There are three possible sizes a butterfly may have. Every butterfly must have exactly one kind of size: Small SizeIcon§Small, Medium SizeIcon§Medium or Large SizeIcon§Large. Butterflies can be attracted by their size by using Attraction Flowers at the Incubator.

Smaller butterflies usually have a shorter incubation time, but also a lower earning rate than bigger ones. There are currently 120 small butterflies released to the game. A list with all small butterflies can be found at the Small Category Page.
Butterflies of the medium size are medium at everything! In the game are currently 148 butterflies of medium size, which can be found as list at the Medium Category Page.
Most impressive and most hoggish are large butterflies, because of their usual big amount of Pollen they need to level up. Currently are 54 large butterflies released. To check which kind of butterfly is large, there's a list at the Large Category Page.

In Forest

The size of a butterfly's egg, caterpillar or chrysalis is obvious to see beside the size icon. You can see the size already at the egg in the incubator, but also at the grow spots when raising.