If that is you and your horse in your profile picture you definitely do NOT need a 17inch seat (or a 16inch seat for that matter). Your horse looks like for a western saddle he will need a saddle that has full quarter horse bars (NOT semi quarter horse bars)! You will have to find a western saddle with full quarter bars and probably a 14 or 15 inch seat for you since you are still small. If you want a cheap saddle that will fit you and your horse.... I'd buy this one Saddles Tack Horse Supplies - ChickSaddlery.com Wintec Western Saddle Full Quarter Horse Bars
I know it's a little over budget but your saddle fit is just as important as your horse and you don't want to get a saddle that hurts him and ruins your expensive horse. Order that in a 14 inch seat and I think you'll be set.