Creature size is an attribute used for several modifiers for feats, skills and abilities. Size also usually determines the creatures "Personal Space" and the height may be useful to know to make more real looking worlds.
The size is linked purely to the appearance ID lines as per below and is not stored on the UTC blueprint and cannot be changed with script commands.
appearance.2da has a few relevant size columns. The SIZECATEGORY links to the sizes in nwscript.nss:
Creature Height and Standard Race Heights
Quick reference these are the standard race heights against an in-game feet guide.
appearance.2da HEIGHT column apparently affects the games pathfinding - yes, surprising everyone when you mention it given it is essentially a 2D pathfinder (you can't have multi-level overlaps of terrain such as a tunnel). The extent of this is relatively undocumented and the HEIGHT values of most creatures is actually just set to 1...mysterious engine!
It also is the height of the player camera (so is why your camera jumps up when polymorphing into a Frost Giant or Dragon), although this might be additionally hardcoded for cases where it is just 1.
You can scale a creatures visual appearance using the transform tools in the toolset or nwscript, and it has absolutely no effect on movement, spell casting, attacks or line of sight.
There are some animations that can change when you change the TARGETHEIGHT column (attacking upwards/downwards, ie a human hitting a low to the ground badger for instance).
Personal Space and Creature Personal Space
appearance.2da has 2 columns for PERSPACE and CPERSPACE. These are related to the radius of the creature.
PERSPACE is the "Personal Space" radius that pathfinding uses. It is the "not overlapping too much of the model" space.
CPERSPACE is the "Creature Personal Space" radius and is always equal or larger than PERSPACE. Attacks are made from the edge of this since creature's hands are outstretched etc. and it might affect pathfinding past creatures. If a CPERSPACE is 1.5, and a short ranged, 8M spell is cast, it can be 9.5 to a point on the ground from the center of the model, or to the edge of another CPERSPACE value. It stops larger creatures like dragons clipping into another creature to attack them in melee.
A few more values like HITDIST and PREFATCKDIST I am yet to fully work out but nwn.fandom suggests they are used to determine melee range.
Examples of values for reference:
|Large||0.6||1.2||1.2||1.9||13, Brown Bear|
|Huge||1.1||2||1.1||3.3||41, Dragon, Black|
Notes on the sizing:
- Huge is generally the only size which will have issues with normal sized doorways, although this may also be down to the height of the creature