DnD 5e - Space and Reach
Each creature occupies a physical space, and this space can't be shared for any extended period of time, though creatures can move through it (see Movement and Position, earlier in this guide) under some circumstances. In addition, creatures have a Reach which extends a specific distance in every direction around them.
Creature's sizes are broken into 6 "size categories". Creatures within a single size category can vary widely in size and shape, but size categories provide a reasonable approximation of the space they occupy in combat. Objects sometimes use the same size categories, especially for items larger than a person could typically carry.
|Tiny||2.5 by 2.5 ft.||House cats, pixies, homunculi|
|Small||5 by 5 ft.||Halflings, goblins, jackals|
|Medium||5 by 5 ft.||Humans, dwarfs, ponies|
|Large||10 by 10 ft.||Horses, Ogres, Lions|
|Huge||Cyclops, Elephant, Adult True Dragons|
|Gargantuan||Ancient True Dragons, Rocs, the Tarrasque|
A creatur's space is the area it occupies in combat. A creature typically isn't a perfect square or cube (with the exception of Gellatinous Cubes), but a creature needs enough space to move about to attack enemies and to defend itself.
Unfortunately, the official Space rules don't discuss creatures' height. If vertical dimensions matter (common when flight is involved), I recommend treating all creatures' spaces as cubes. This means that even massively tall humans still somehow need just 5 feet of vertical space to fight effectively. This doesn't make a lot of real-world sense, but this is a game and sometimes realism needs to be sacrificed to make the rules usable. I remember 3rd edition's attempt to differentiate between "long" and "tall" creatures, and it added nothing of value to the game.
Squeezing Into a Smaller Space
Sometimes a creature doesn't have as much room as they would like. Squeezing into narrow hallways, crawling through chutes, or being squeezed into a room with too many occupants all happen from time to time.
In these cases, a creature is "squeezing". A creature can squeeze into a space that is large enough for a creature one size smaller than it. For example, a large creature could squeeze through a passage that's 5 feet wide, which is enough for the space of a medium creature.
While squeezing, movement costs 1 extra foot of movement for each foot traveled. This additional cost stacks with others, such as difficult terrain. In addition, creatures that are squeezing suffer Disadvantage on attack rolls and on Dexterity saving throws. Attacks rolls made against creatures that are squeezing have Advantage.
Most creatures have the ability to reach beyond the space they occupy. This allows creatures to interact with items, attack, etc. With some rare exceptions, playable races have 5 foot reach, meaning that they can reach and attack anything within 5 feet of them.
Reach is always measured in 5-foot increments, and varies wildly between creatures. Creatures that can reach far away from their bodies will have long reach, while even gargantuan creatures might have short reach because their limbs are short.
Some weapons, such as glaives, have the "Reach" property. This allows the wielder to use the weapon as though their reach were 5 ft. longer than normal. This benefit applies as long as the wielder continues to wield the weapon, so they reach remains extended during other creature's turns.
Reach on a Grid
While playing on a grid, measure reach the same way that you measure movement. 5 feet of reach allows a creature to to attack enemies one square away, while 10 feet of reach allows 2 squares, and so on.
If you use the "Diagonals" optional rule on page 252 of the Dungeon Master's Guide, you'll notice a peculiar issue with reach weapons. When measuring a creature's reach diagonally, you'll find that a meadium creature with 10-foot reach can only reach one square diagonally because the second square counts as 10 feet. This means that the easiest way to get into/around/through the reach of such a creature is through the corners of the area that they can reach. If this proves to be a problem in your game, you can address it by agreeing to round up when measuring reach instead of down as you normally would.