Player characters have an average height determined by their race, which ranges from about 3 ft. tall (Burghal Gnomes) to 7 ft. tall (Giantmen). They can additionally customize their appearance when viewed by other adventurers (see Character Guide for Height and Age), so that they can be "taller than average" for example, but this is purely cosmetic and not known to have any mechanical effects.
Creature height is much more variable, ranging from only 1 ft. tall rodents to massive titans many times the height of even a giantman. Creature height can be checked with the APPRAISE command (or equivalently ASSESS).
When attacking or ambushing with a melee weapon, shorter characters will not be able to reach the head or eyes of much taller creatures (unless they are knocked down: creatures who are kneeling are considered to have only 60% of their usual height; if sitting, only 45%; if prone, only 10%). If the creature is exceptionally tall then it may not be possible to reach even the torso. The body areas in order of descending height are: Head ≥ Eyes > Neck > Chest > Arms > Back ≥ Abdomen > Hand > Legs. Longer weapons can allow a character to hit an area that would not otherwise be reachable: for example, a morning star adds 2 ft. to the attacker's effective height, and a lance adds 5 ft.