Endurance saddles, if well made, ARE much more comfortable than regular saddles. People sit in them for 100 mile rides...they have to be. I've even competed in a 50mile in one of the cheaper ones (Hilason to be specific) and was not sore at all. I rode for 3 hours in a regular western saddle yesterday (a NICE one, just not for endurance) and my back and butt have not stopped hurting...I'm in SOO much pain! Now, that's unusual for me. I ride competitive trail and when it's nice out I ride 2-4 hours at least every other day. I don't have any pain bareback or in endurance saddles.
A basic rule of thumb for treeless saddles, is that they are not good for a horse with high withers. They will put too much weight on the withers, and that's bad for obvious reasons. Other than that, they will fit. They don't have a tree, so they don't have their own shape. Therefore, they can't really be the "wrong size", they could just be the wrong type of saddle. That being said, not everybody likes them. They do lack support, and can feel unstable at first. You'll feel more stable if you get one with a suede type seat, or if you use a sheepskin cover to keep your butt in place. Treeless can be slippery otherwise. You should try one out from a local tack shop if you can, and see how it works for you. If you like it, order one online :)
OH! Also, something to remember in a trail saddle, it to get one that has a low horn or no horn. I don't mean english, because you already stated that wouldn't be comfortable for you...but one that has a horn can be bad when you need to lean over the horse's neck and you get gutted