Try running a peice of string from where you want the girth to buckle on one side underneath the pony to the girth point on the other then measure it.
Your girth length will vary not only with how stocky pony is but also where on the saddle the girth staps sit and how far up them you want the girth the fasten. (Don't aim to have them tight only on the top hole as the girth will stretch and you'll have no where else to go!)
