Monster Sizes

Although you should never use one of these as an actual monster size, except for on minions and the Huge and Epic creatures, these are a good guide. Monster Sizes are as follows,

Tiny: Anything that is much smaller than a human (i.e. a Hornetaur or Melynx).
Small: Anything that is smaller than a human but larger than a small creature (i.e. a Kelbi or Mini Kut-Ku).
Medium: Anything that is around the size of an average human (i.e. a Blango or any non-alpha Raptor.)
Big: Anything larger than a human but smaller than a large creature (i.e. a Yian Kut-Ku or an Alpha Raptor).
Large: Anything much larger than a human (i.e. a Rathalos or Blangonga).
Giant: An extremely massive creature (i.e. a Plesioth or Gravios)
Huge: A creature so massive it takes a fortressjust to slow it down (i.e. a Shen Gaoren or Fatalis).
Epic: A creature so large that a single bite could swallow a human whole (i.e. a Lao-Shan Lung or a Navaldeus)

the measurements used in the comprendium is measured in units of Hands (i think)