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"There are probably more effects of size not here supplied. Further, the article could use clean-up and citations."
In NetHack, all monsters are classified in six distinct sizes: Tiny → Small → Medium → Large → Huge → Gigantic. Each of these map to an integer used, for example, in determining how much the monster can carry: Tiny = 0; Small = 1; Medium = 2; Large = 3; Huge = 4; Gigantic = 7. Some types of weapon distinguish between "small" monsters (tiny to medium) and "large" ones (large to gigantic).
Only monsters medium or smaller can wear cloaks, and only exactly medium monsters can wear shirts or armor. Size also restricts which monsters can move diagonally through a crack. Statues of tiny monsters cannot contain spellbooks; for details on the rest, see the miscellaneous items spoiler.
Huge monsters cannot be engulfed, while large ones can.
Size is only one component that is checked when determining if armor and other items can be worn. For example, non-humanoids will always break or slip out of torso armor, depending on their size, and even some medium-sized humanoids will break out of armor for shape reasons. The code currently has two examples of this, the winged gargoyle and the marilith (wings and too many arms, respectively). One effect of this is that there is no armor-safe metallivore for polyself purposes.