Perhaps showing my age here, but in "The Good Old Days", 10-foot squares were a standard map size. Each square on the map was 10 feet, and a 10-foot wide corridor was normal.
Now, having started gaming again, it seems 5-foot squares are standard.
So my question is: when did this change happen? Was it 3rd edition?
The five-foot square was not standardized on until D&D 3rd edition, which made it a standard part of the rules in the original core rulebooks published in the year 2000. However, 10ft and hybrid 10ft/5ft squares still appeared in some dungeon maps until the D&D v3.5 revision (2003), which encouraged the designed encounter areas around the use of the 5-foot square combat grid.
The AD&D 2nd edition adventure module Die Vecna Die! (2000), one of the last AD&D books published, still uses 10-foot squares for its dungeon maps, including the map of Adytum on page 20, and the map of Vecna's Palace on page 63. Some maps of large outdoor areas use different scales, such as 25 feet or 30 feet.
The original D&D 3rd edition, nowadays retroactively known as 3.0, made the 5-foot square standard. The 3.0 Dungeon Master's Guide (2000), p.67, "Scale and Squares", states:¹
The standard unit for tactical maps is the 5-foot square. This unit is useful for miniatures and for drawing dungeon maps, which are usually created on graph paper.
In a fight, each Small or Medium-size creatures occupies a single 5-foot square.
However, the sample dungeon map on page 127 still uses the traditional AD&D a scale of 10 feet.
The Player's Handbook v3.0,² inconsistently, does not explicitly use squares, but instead recommends a scale of 1 inch to 5 feet for miniatures at the table (perhaps assuming that while dungeon maps may be gridded, combat will not necessarily be played out with the luxury of a one-inch grid on the gaming table, but instead measured with rulers like a miniature wargame). However, it does specify that Small and Medium creatures have a 5-foot facing, which is the space they occupy in combat.
The D&D 3e adventure module The Forge of Fury (2000) does not mark a scale on its maps, but based on the description of area 9 (30x15 feet for a 6x3 square area), it uses 5-foot squares. This suggests that the mapmakers could assume that five foot square area in D&D 3e was a given. The Speaker in Dreams (2001) also uses 5-foot squares.
Some D&D 3e adventure maps used a kind of hybrid 5-10 foot square, with larger 10 foot squares subdivided into 5-foot squares, or five-foot squares with thicker lines for 10-foot increments. The Sunless Citadel (2000), Heart of Nightfang Spire (2001), Deep Horizon (2001), Lord of the Iron Fortress (2002), and Bastion of Broken Souls used this. Return to the Temple of Elemental Evil (2001) also used this approach, stating "One Large Square Equals 10 Feet".
Dungeon magazine was inconsistent. Dungeon #82, the first 3e issue, uses 5 foot squares in some maps, 10 foot squares in others. Dungeon #95 (Dec 2002) still used some 10 foot square dungeon maps. Lost Temple of Demogorgon, Dungeon #120, used 10 foot squares. Even the last print issue, Dungeon #150 (Sept 2007), still used some 10-foot square maps for very large open areas, such as the Battle of Wat Dagon.
D&D 3.5 (2003) explicitly assumed the use of miniatures and a "1 square = 5ft" scale battle grid (PHB p.4, "Three Dimensions"), with an eye to encouraging the sale and use of the company's miniatures products. The Dungeon Master's Guide v3.5 came with a paper battle grid, and all monster stats thereafter listed movement in 5-foot squares in addition to feet.
This solidified the use of the five foot square in dungeon maps. For example, all indoor maps in Expedition to the Ruins of Greyhawk (2007) use 5 foot squares.
¹ Dungeon Master's Guide, Second printing, September 2001.
² Player's Handbook, Second Printing, November 2002.
The 5-foot square is the standard measurement in every edition of D&D published by Wizards of the Coast, starting with 3rd Edition in 2000.
In editions prior to WotC taking over publication, there was no standard. Mapping of constructed areas was typically in 10-foot squares, and the ubiquitous gridded battle mat was almost unknown to D&D players. The 5-foot square and gridded combat made an appearance in the TSR-published Player's Option: Combat & Tactics in 1995, and this might have influenced later 3e design, but it wasn't widely adopted and can't be said to have made the 5-foot square a standard.
Prior to moving to a visually-focused, grid-based design, D&D editions typically simply gave limits to how many combatants could fight abreast in a given corridor width. The most common rule across editions (I believe) is that three combatants can fight side-by-side in a 10-foot corridor. This 3-foot-wide clearance wasn't intended to describe how much space a character occupied or controlled in combat though – it was exactly and only a statement about how close friendly forces could pack together while still being able to fight.
Because earlier editions had a heavy dose of simulationism, abstractions were either avoided and whatever non-standard measurement that was appropriate was given directly, or the abstractions were at a more more granular level than anything that could be consistently gridded. Measurements were also were very context-dependent. In other circumstances than combat, the space occupied by characters was described differently – for example, when squeezing through narrow caves, the Dungeoneer's Survival Guide (1986) gives a table of minimum tunnel widths by race, listing humans as requiring at least 2 feet of width to pass and elves needing 1½ feet, as well as variations in minimum height clearance.
As a general rule, TSR editions of D&D (and games based on them) can be assumed to use continuous real-world measurement systems for everything, while WotC editions can be assumed to use a discrete 5-foot square. This continuous/discrete paradigm difference is fairly fundamental in their respective designs, with the measurement style coming first, with rules built on top and referring back to the measurement paradigm. For example, D&D 3e and later mostly bases movement rules on the 5-foot square and on manipulations of whole or part squares, while 2e and earlier deal in yards, feet, and inches and modifications to those continuous measurements.
According to some basic searches, it seems the battle grid of 5 feet seems to have been used a lot during 2nd edition, but it was made the standard during 3rd. And deeply codified by the time of release of D20 SRD.
Keep in mind that the grid is a variant rule in 5e, and isn't technically required for a Rules as Written game. The grid is what you make it as DM. I often play without a grid, or with a 5ft grid. But I sometimes even go smaller for things like mazes, using Lego Bricks to represent the characters where 1 stud = 5 feet.
As far as the passages in old maps goes, it really depends on if you want to allow the players to go through two at a time or force them single file. Both have their places, and some maps might not matter in some instances (just connecting rooms) and others it might be really important (party looking traps = rogue in front or afraid of ambush = tank in front).
In support of other answers, here are some additional historical notes about D&D's tabletop scaling factor (which may be related but necessarily the same as standard map squares).
In 1974, Original D&D was printed with the specification that 1" = 10 feet in dungeons (and 1" = 10 yards outside). Articles from Dragon magazine assert this was a continuation from the custom in Dave Arneson's Blackmoor games. But in that same year TSR also published two other games that both used 1" = 6 feet scales: the western Boot Hill and Warriors of Mars (a game simulating Burroughs' John Carter of Mars books, which are referenced in numerous places in the Original D&D books).
In 1980, the Tom Moldvay Basic D&D set was published. At one place (in the Encounters section) this book specifies that for table top miniatures one should use the traditional 1" = 10 feet; but later in the same book (the final Notes to DMs section, likely Moldvay's personal modifications), it instead recommends that 1" = 5 feet be used. This is the earliest D&D-branded publication suggesting that scale of which I know. (Note also that if directly converted, the standard 28mm or so scale miniature is very close to a 1" = 5 feet scale.)
The AD&D 1E system officially stuck with a book 1" = 10 feet indoors scale, but note that this had become a unit-of-account only, not a tabletop scale. The introduction to the 1E DMG suggests instead that a tabletop scale of 1" = 3⅓' should be used.
It was indeed 3E that finally, uniformly established a miniature scale of 1" = 5 feet, and some of the mapmaking resources took that as the map grid size for easy transfer to the tabletop.
You can trace the origins of this 5' tactical based sizing to well before A&D 3rd and back in the days of AD&D Basic and 1e AD&D. Many of the maps produced had overlays, but crucially for me were GW's dungeon floorplans and the like that we would lovingly piece together into maps and dungeons in careful reproductions of the maps that were produced.
These early editions of AD&D didn't have the idea of square by square movement, but certainly the floorplans and maps were the seed that created it.
Once 3rd Ed hit this 5' became codified into a more tactical aspect with ideas such as the 5' step and movement broken down into 5' chunks.
(This was a bit too big for a comment and I wanted to post the link for the floorplans!)
As far as I can tell, the idea of a character occupying a 5' by 5' area started in 3rd ed D&D.
Neither Holmes nor Mentzer Basic mention the idea of facing or area occupied at all (I have no Moldvay Basic for comparison). There is mention in Holmes Basic of D&D often using a scale of 1"=10' for miniatures, but that's about it.
AD&D 2nd edition DMG mentions how many attackers can engage a single enemy (p57 - 6, by the way), but no amount of area occupied. Creatures in the Monstrous Compendium have a Size listed, and often list a specific length or height (e.g. Carrion Crawler is L (9' long), while a Hill Giant is H(16' tall). Still, no mention of grids, or use of squares or area occupied.
3rd ed D&D PHB, on p130 and 131, lists the Standard Scale as "One inch = 5 feet", and "A human-size creature occupies an area 1 inch (5 feet) across)". So, I would count this as the first real mainstream use of the 5'x5' area. Bear in mind, though, that even in 3.0, you could play without a battlegrid.
4th ed D&D standardised on using squares as the unit of measure.
