Size Large

In Dungeons and Dragons Firth Edition (D&D5e), horses are considered to be of large size. Well Centaurs are considered Medium size. Does this mean a lot of difference in perspective to game play? No. What it does mean is representing a character (or creatures) facing.

Medium creatures are thought to fill a 5 foot by 5 by 5 foot space
Large creatures are thought to (mostly) fill a 10 foot by 10 by 10 foot space
Huge creatures (mostly) filling a 15 foot by 15 by 15 foot space

Size

A monster can be Tiny, Small, Medium, Large, Huge, or Gargantuan. Table: Size Categories shows how much space a creature of a particular size controls in Combat.

Space and Surrounding

Creatures of different size occupy different amount of space (when on a battle maps). The Player’s Handbook defines space as “the area in feet it effectively controls in combat”. This means a creature isn’t five feet wide (small or medium size); that’s just the space it can comfortably act in without needing to move.

Most creatures are Medium sized (5 foot by 5 by 5 foot). Lets say the PC is a 10 ft tall Goliath, they still would be considered medium as they take up 5 foot by 5 by 10 foot.

Horses would be 5 by 10 by 10, and could be argued as being either Medium or Large – but because we commonly ride them its better to have them as large – 10 by 10 by 10.

Centaurs being a playable race, they are made to be Medium in size for convince – you don’t want you’re PC to have a 10 by 10 facing, it makes it to easy to be surrounded and makes moving through spaces to difficult.

This is because a creature’s size defines the space it comfortably takes up, and also sets the limit of how many foes can surround them in combat. For example: a medium fighter takes up a 5×5 square (or hex) on the battlemap. If all the attackers are also medium (or small) size, eight attackers can surround the Fighter, each taking up their own 5×5′ space.

The larger the creature, the more spaces surround it, so the number of (commonly smaller) creatures able to surround it is also higher. Few larger creatures can surround something smaller.

D&D5e doesn’t have a system of differentiating the half sizes (you’re either a size or not) and the only mechanic for size is facing (and dismounting if you’re Mounted)

Squeezing

All creatures can squeeze through gaps on size smaller then themselves (commonly). A large creature can fit through a medium space, medium through a small one, etc.

Your DM may tweak this to use common sense (comparing height and weight which has no relation to the size rules as written).

Blocking

Creatures need to be at least two sizes different for hostile creatures to move through each other’s space. Example: a medium Barbarian can’t move through a large monster’s space, but they could through a huge foes area. Just as a Barbarian would struggle to block a tiny enemy from getting past.

Grappling

Creatures can only be grappled by a monster one size smaller than them (“No, you cannot grapple the Tarrasque!”). Halflings can only grapple medium foes, not large.

Weapons and Armour

Weapons have many properties, one being “Heavy”. Heavy weapons are bulky for medium and most large creatures, which means small and tiny creature struggle to use them. Any creature small or tiny fighting using a Heavy weapon have disadvantage on all attack rolls.

Armour has little in the way of official rules, a smaller creature could wear any armour. A DM may impose rules on equipment size. The Player’s Handbook calls these “common sense”, so use that as you may.