All suits of armor are built for a particular body shape and size. Armor is most often made in the shape of a human or demi-human. The table below describes the proportion of armor sizes found at random. Within the general size, the armor may be adjusted to fit individuals of average variations. Armor made for humans may not be worn by dwarves or elves. Shields used by humans may be used by dwarves or elves. Halflings must use their own half-sized shields, and giants use larger shields.
As the bodies of humanoids typically mirror that of humans, those of similar size can wear armor of the appropriate size. For instance, a hobgoblins and a human can wear the same suit of armor. Likewise, a halfling might borrow a suit of goblin armor. A suit of armor made for a lizardman likely will only be of use to lizardmen.
Unless otherwise specified, magical armor does not resize to fit different body shapes or sizes. Some suits of magical armor could have this feature, of course.