Thanks for all of you that were helpful. I figured it out. My horse right now needs a 25" girth. I have a 28" girth, and is quite a bit big. So, I'm going to get a 26" (24" would be too small, plus she's growing still, and, there is no 25").
Still don't know the "math". Yes, down under is a good site, but their smallest girth 28, and as it is, I had to poke extra holes in the strap to make that one fit! Considering that the rig to get my saddle "converted" would cost more (about 100$ maybe a bit more), I'm going to deal with the 30$ girth, since my only issue is it being too big (and horse is a bit sensitive), I'm not having difficulty with cinching.
What they do with ponies in Australia? Eat them? Jeez. Beware if you got a little horse! Hahaha.... In all fairness, my horse and I both enjoy our aussie. It's the go-to guy (though, occasionally will swap over to english or western for whatever reason. RARELY use my western now- I haven't found anything it does better... )