Originally Posted by pkelchner
I have a TB and I bought a 50 girth for her and I still have some extra for growth as well. Mine are also a Mohair and the other is wool fleece. That is English not western. It depends on the saddle.
if we're discussing the horse and saddle in the picture the OP provided, she will need a SHORT dressage girth, not a long english/hunter/jumper/all-purpose girth. In which case a 50" girth will not work.
I do believe the recommended conversion for this would be to take the long girth length and subtract by 20". So for a horse that takes a 50" long girth, in theory, a 30" short girth should work. Of course saddle type/shape and billet length needs to be taken into consideration, but that adjusted measurement seems to be a pretty good place to start.