44 is about average, but they definitely make bigger. I've known horses who take a 52 or 54 girth. Those are BIG animals!
JJ and Casper both take a 42. JJ's bigger than Casper, but Casper has a large barrel.
I always buy girths with elastic on both ends, so they stretch farther than their actual length. Which is why JJ and Casper can wear the same girth.
If you can, measure the horse. Place the saddle and pad on the horse, and using a tape measure, measure from the 3rd hole on one side of the saddle's billet strap, under the horse's belly, and up to the 3rd hole on the other side. That should give you a fairly decent idea of the girth size.
Different saddles take different size girths. Dressage saddles for instance, always take shorter girths because the billets are lower.