Shadow monks and shadow sorcerers are able to teleport from one area of dim light or darkness to another.
What would you say is the limit of the area of dim light? For example, by using a large blanket to create a shadow or if they’re wearing full body clothing like a burqa they’re technically always in a shadow. If the above examples could work, should they be allowed to bring it with them as they teleport?
What would you say is the limit of the area of dim light?
Firstly, to answer what defines the "area", we must first identify what "dim light" is -
Dim light, also called shadows, creates a lightly obscured area. An area of dim light is usually a boundary between a source of bright light, such as a torch, and surrounding darkness. The soft light of twilight and dawn also counts as dim light. A particularly brilliant full moon might bathe the land in dim light.
"Shade" is not the same as "shadows", particularly in this instance. For example, this would not be possible in the middle of the day, in the middle of the street, by simply flinging a blanket over you. In the "dim light", you can make out silhouettes and shapes, but only if you look carefully.
On the other hand, standing between two lights sources, one might be able to clearly see your silhouette, but be unable to make out any detail, as you are too far away from the light. In this case, it'd be perfectly viable.
As for exactly how big this area need be - well that can be defined by the DM. How far light may travel is more scientific, and while definable, would merely be playing with semantics. As many other questions have found, physics and D&D don't often mix well.
Here is the relevant wording of the Way of the Shadow monk's Shadow Step:
At 6th level, you gain the ability to step from one shadow into another. When you are in dim light or darkness, as a bonus action you can teleport up to 60 feet to an unoccupied space you can see that is also in dim light or darkness
The core of your question is if the monk can create their own area of dim light by using a blanket or clothing they wear or carry. That does not work as their clothes and equipment teleport with them, and therefore also must be in an area of dim light, which they will not be if the overall area is not dim or dark. Some spells and effects (like this one) that teleport you do not explicitly say they teleport you with the gear you wear and carry, but there is a Sage Advice Compendium entry on it:
Misty step doesn’t say the caster can bring worn or carried equipment with them. Are they intended to leave everything, including their clothes, behind? No, the caster’s worn and carried equipment are intended to go with them.
Some teleportation effects do specify that you teleport with your gear; such specification is an example of a rule being needlessly fastidious, since no teleportation effect in the game assumes that you teleport without your clothes, just as the general movement rules don’t assume that you drop everything when you walk.
So for the purpose of teleportation, the clothes are treated as part of the character, and the entire character needs to be in dim or dark light. Therefore, you cannot use them to create the shadow conditions you need. You must be in an actual area of darkness or dim light.1
The target area is described as an "unoccupied space". Outside of combat, an area small enough for you to physically fit in might be sufficient. In combat situations, space is defined on page 191 PHB:
A creature's space is the area in feet that it effectively controls in combat, not an expression of its physical dimensions.
and a table on that page shows that the space of a Small or Medium creature is at least 5 by 5 feet large. That means you need at least a 5 by 5 feet area of dim light or darkness as the target space to teleport to in combat.2
While the ability does not state that the starting area is a space of dim light or darkness, in any combat situation for practical purposes you will be in such a space, and will have to make a call if that space the character is in counts as dim light or not. If you use a grid, most light sources have a circular range in multiples of 5 feet, and the DMG gives you this rule (page 251):
Choose an intersection of squares or hexes as the point of origin of an area of effect, then follow its rules as normal. If an area of effect is circular and covers at least half a square, it affects that square.
so if more than half of the square is in darkness or dim light, you can teleport from the square. If they are not, you cannot. If you are not using a grid and stick to narrative combat, then the DM will have to tell you if the area you are in is sufficiently dim or dark to teleport from.
1 There are ways to create the conditions you need in a way you can carry. For example, you can cast darkness (which you conveniently know as a class ability) on an object and carry it around with you, and you can even put it into a bag or other container, to suppress the emanating darkness while you do not want it. The problem with that is that you need to see the space you teleport to, and normally you cannot see through magical darkness, but there are ways around this, too.
2 You can question if an unoccupied space must be the same size as the space controlled the teleporting creature. I think in the absence of other definitions of space, this seems to be the most natural interpretation, as a creature's space and occupied space in combat are the same, and you also will turn that space into an occupied space by teleporting there. Outside of combat, you might get away with a smaller dim or dark space.
This related question is similar, but extends the scope of how to deal with the PCs setting up a small tent and moving inside to trigger dim light or darkness conditions. I think that would be legitimate. In combat, setting up a tent is impractical, so this tactic would be relegated to exploration time.
It may be tricky to enable the Shadow Step feature with it, you'd need to set it up just right to block enough light so you can be in dim light but still look out through an opening, as you otherwise would not be able to see the target space. But triggering other features, such as the Twilight cleric's Steps of the Night feature that have no line-of-sight requirements should work fine.
