How tall is your horse, and about how much do they weigh? What breed? I don't know if anyone will be able to give you an exact size, but I just went through the same thing, and may be able to help narrow it down
I guess my other suggestions would be to measure your current one, from the end of one metal buckle to the other and see what size it is, and go from there. Not sure how far you are from the tack store, but if you're close enough, you could always buy 2 (or 3) and return the ones that don't fit. I bought a 32" for my son's mare, and it was too small, so I exchanged it for a 34" and we're good now.
