I have somewhat thuoght about said size / size difference.
Where I would see most mamono fairly close to average human, say, 5'2" to 6'5 Some are petite, and some, althuogh not that many are taller / larger, and most are in between, as of individuals.
But then there is few species, that on average are smaller or larger,
Such as Minotaur who I would see being petite, when she's under 7 feet, but it's more common to run in to some that are closer to 8 feet, some might be slightly taller than that.
Holstaur and Hakutaku being slightly smaller
Ogre/Oni that are slightly taller than that,
As I don't like the the art of arachne, I would see the non human parts being larger, the back part of the body a bit more bulbous, bit like Rachnera in monster musume, thuogh not as large.
Lamia, and every other nake like creature, In art they seem short, (and that's coming from someone who has snakes.) the snake body I would expet to be well over 35 feet long, Assuming the human part is rather petite.