Originally Posted by MarleyandEllie
But like, you know the 15" 16" 17" saddles? What is that measuring? And how do you find out what your horse needs.
That would be your seat size. And it's for -you- not the horse. As in, your butt may fit in a 15" saddle while others fit in an 18" Saddle. You can get a general idea from this: English Saddle Seat Calculator
For your horse, you need to look at their withers and their back. Some horses have high withers so they need a cut-back pommel, etc. For example, a horse with high withers wont fit easily in a saddle with a wide gullet (part near the pommel), as it will just lay on their withers. And then you also have to see where the saddle is pinching them, is the tree too wide or too narrow on their spine, etc. Its a little complicated to explain over the internet for me. You can always post pictures of your horse on here and ask advice, or put a saddle on your horse and ask for advice for it it fits or not.