What size category is a reduced Tarrasque?

Question

The rule for a creature's size is as follows on Monster Manual page 6:

$$ \begin{array}{lll} \textbf{Size} & \textbf{Space} & \textbf{Examples} \\ \hline \text{Tiny} & \text{2½ by 2½ ft.} & \text{Imp, sprite} \\ \text{Small} & \text{5 by 5 ft.} & \text{Giant rat, goblin} \\ \text{Medium} & \text{5 by 5 ft.} & \text{Orc, werewolf} \\ \text{Large} & \text{10 by 10 ft.} & \text{Hippogriff, ogre} \\ \text{Huge} & \text{15 by 15 ft.} & \text{Fire giant, treant} \\ \text{Gargantuan} & \text{20 by 20 ft. or larger} & \text{Kraken, purple worm} \\ \end{array} $$

The Monster Manual page 268 states:

A scaly biped, the tarrasque is fifty feet tall and seventy feet long, weighing hundreds of tons.

And the Reduce use of the Enlarge/Reduce spell states:

The target's size is halved in all dimensions, and its weight is reduced to one-eighth of normal. This reduction decreases its size by one category - from Medium to Small, for example.

Emphasis mine.

In the case of our Tarrasque, these statements in Reduce are contradictory. Its new size is now 25′×35′, which means it's still classified as Gargantuan.

Which rule takes priority?

We can break the rules even more here. If we assume the Tarrasque is now huge rather than gargantuan, PHB 192 states:

Squeezing into a Smaller Space: A creature can squeeze through a space that is large enough for a creature one size smaller than it. Thus, a Large creature can squeeze through a passage that's only 5 feet wide. While squeezing through a space, a creature must spend 1 extra foot for every foot it moves there, and it has disadvantage on attack rolls and Dexterity saving throws. Attack rolls against the creature have advantage while it’s in the smaller space.

This means our 25′×35′ beastie can now get through a 10 foot door.

Answer 1

This is one of those edge cases where a DM would need to make a ruling.

Given that the Tarrasque is exceptionally large, it would be reasonable for the line

This reduction decreases its size by one category

to take precedence over the line

The target's size is halved in all dimensions

In other word, the Tarrasque is reduced by more than half such that it can be considered a Huge creature, rather than still a Gargantuan creature. Given that magic has been used to reduce the Tarrasque (meaning that presumably a resource has been expended and the Tarrasque has failed a save, etc), it seems fair for the Tarrasque's opponents (presumably the party) that this should be the case.

Of course, it may also be reasonable for a DM to say that the Tarrasque is so large that reducing doesn't reduce its category, so the lines above take precedence the other way around, leaving the reduced Tarrasque as still being a Gargantuan creature.

Ultimately, without a Word of God answer, this one is up to the DM.

Answer 2

This may not be that contradicting at all.

The Tarrasque is indeed getting reduced to a 35 ft long, 25 ft high creature. It may seem odd to have it be considered only huge having these dimensions. However, consider the following comparisons:

Length

There is precedent for creatures longer than their size category would suggest namely the shark family:

1. The reef shark is said to be 6 to 10 feet long and is medium (5x5 space)
2. The hunter shark is 15 to 20 feet long and is large (10x10 space)
3. The best comparison is the giant shark which is 30 feet long and huge (15x15 space)

Another example (albeit not exactly about "length") is the pteranodon which is medium and sports a wingspan of 15 to 20 feet.

Height

In terms of height, compare with most humanoid player races which are medium but have more than a 5 ft height as well. See goliath for one of the more extreme examples (6 to 8 ft height as a medium creature)

Perhaps a better example for height is the Goristro, said to be "more than twenty feet tall" and being a Huge creature.

Conclusion

Comparing with these other existing creatures the Reduced huge Tarrasque becomes much less of a stretch.

The problem may be that the reduced Tarrasque is combining both length and height bigger than the Huge size dimensions which may be an argument for making all these comparisons not applicable. I still believe it is something useful for the DM to consider when deciding how to deal with this situation (see Nathan's answer)

Answer 3

The reduced Tarrasque is Huge.

There is a subtle distinction between a creature's dimensions and its size. The dimensions of a creature are rough indications of its body's shape, whereas the size of a creature defines how much space it controls on a grid. The dimensions and size are often related, but not strictly so.

A creature's space is the area in feet that it effectively controls in combat, not an expression of its physical dimensions. A typical Medium creature isn't 5 feet wide, for example, but it does control a space that wide.

This part of the reduce spell alters the dimensions of the target's body:

The target's size is halved in all dimensions, and its weight is reduced to one-eighth of normal.

This part of the reduce spell alters the target's size category and therefore changes the space it controls on grid:

This reduction decreases its size by one category — from Medium to Small, for example.

Hence, the reduced Tarrasque is a Huge creature and controls a 15' × 15' space on a grid, regardless of the dimensions of its body.

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The Squeezing rules do not work well for creatures with extreme dimensions, if you imagine them as static figures. A classic example is a snake, which ought to be comfortable in very narrow spaces relative to their size. However, the Squeezing rules are best understood as a kind of inconvenience for combat, and it's part of the DM's job to narrate these kinds of effects.

A large snake might fit comfortably in a narrow hallway, but it will have trouble turning its head to bite, or gathering its body to constrict. The Tarrasque has the opposite 'problem': it doesn't fit comfortably in a relatively narrow space, but it's so overpowering that it can still plow through it. After all, the Squeezing rules don't say that a space survives unharmed after a creature squeezes through it.

Answer 4

In 5e, specific overrules general. However, which is the specific might be up to DM discretion.

The following would allow the benefit of Reduce without contradicting size:
I would say the Tarrasque is still gargantuan, but half as gargantuan as he was, so reduce its damage due to the size reduction.

Reduce specifies:

Until the spell ends, the target also has disadvantage on Strength Checks and Strength Saving Throws. The target’s weapons also shrink to match its new size. While these weapons are reduced, the target's attacks with them deal 1d4 less damage (this can’t reduce the damage below 1).

Answer 5

In my semi-professional opinion a Tarrasque is technically a size class a step above gargantuan; Its manual entry also notes this via the "(titan)" in the size line.

As such it's been my practice as a DM to reduce it down to simply "gargantuan" if it happens to be affected by a Reduction spell, applying all of the appropriate negatives of the spell otherwise.

Ultimately it is up to each DM to determine what is the best fit for their setting though.

Answer 6

Gargantuan starts at 4". I would put the tarrasque at 8" (gargantuan) to start with, and when reduced, it becomes 4" (the smaller end of gargantuan).