Have you ever wondered how a designer decides on the size of the finished picture? For my designs, it comes down to fingers! Since everything is outlined in black and then colored in -- a feature common in medieval art -- all my fingers require 3 squares: one flesh-colored for the finger and one in black on each side to outline it.
Sometimes delineating all the fingers will make the final design just too big, in which case the poor person will be relieved of a finger or two.
Feathers, like fingers, can only be reduced so far. The narrowest feather needs the same space as a finger and will thereby determine the final size of the winged creature.
Beyond fingers and feathers, a design more or less tells me what size it should be, what size suits the proportions of the original, what size is not so huge that someone would never stitch it. For example, for my own pleasure I am working on 1/3 of a woodcut picture of Basel which is going to be rather large assuming it ever gets finished. I have stitched Very Large designs and enjoyed them, but for the most part I think we all like something that we can count on finishing in a reasonable amount of time, yes?