It's been a while since I bought a saddle, but now with a new horse, I need to go saddle shopping again. Plus, I am thinking of replacing my first horse's first saddle, which he has worn since he was a 3-year-old -- it still fits him great, in fact it probably fits him better since his back lengthened (the picture above was taken when he was three), but I don't like the thickly padded flaps of the all-purpose English saddle. He isn't very responsive to leg pressure when I'm riding in the saddle, whereas he responds well bareback, so I have been wondering if a close contact saddle would be an improvement for both of us, especially since we are starting to jump a little more now.
Since it seems like my horses aren't going to be able to share saddles -- my new one is a draft cross, and although he is only three now, I have a feeling he is only going to get wider as he grows and fills out -- that means I am currently in the market for not just one, but two saddles.
The first step, of course, will be to do wither traces of both horses, and to measure my first horse's current saddle (I think it's a medium wide tree, but I can't remember for sure). For help fitting an English saddle, here is a wonderful post on the difference between tree size and gullet width (it's not the same thing), and another helpful page with a template to help you determine tree size. Keep in mind, of course, that the template given is for Thorowgood saddles and other manufacturers might have different specifications for their tree sizes; but it should at least help you get a general idea of your horse's size.
As for how the saddle fits you -- English saddles will always specify a seat size, which is measured from one of the little nail heads on either side of the pommel, to the center of the cantle (the high point of the very back of the seat). This is the measurement that needs to fit you -- this has nothing to do with how the saddle will fit the horse! For instance, the saddle I currently ride in has a 16.5" seat, which fits me well -- 17.5" and even 17" is way too big. But I'm pretty little, so most adult riders will require a larger seat than what I ride in.
By the way, don't forget that this information is for English saddles -- Western saddles are different. For instance, the seat is measured differently, so that if you ride in a 17" seat in your English saddle, you may ride in a 15" or 16" in a Western saddle.
Finally, bargain hunting on Craigslist or eBay may help you to get a good saddle for an even better price, but only if you are positive that you have the right size -- otherwise you may end up with a saddle that doesn't fit, and a seller that won't accept returns. Tack shops and saddle fitters are generally willing to accept returns within a certain time frame if the saddle does not fit your horse, so if this is the first time you have bought a saddle, or the first saddle you've gotten for this particular horse, you may want to stick with a shop that has a return policy!