I don't want you to get the impression that I'm a bear expert. I don't live in bear country, so only get 4 or 5 a year. Also there's as many ways to do taxidermy as there is to build bows, so I'm sure someone will read this and probably have different advice...
It's a bit of a guessing game. If it's in the pickle or even tanned it shrinks approximately 25%. Once rehydrated with water it will stretch if it's thinned properly. I'd wait to your done tanning, measure it flat and go one size bigger. If it measures 4inch, get the 4 1/2 in form. etc... that should be really close. I generally have a few sizes to test fit, and that an option, order 2 sizes you think will fit and return the one you don't use.
For most of my rugs hot glue if they want flat felt... if they want a pleated look I send them off to be sewed at about $60 dollars a foot..😳