Tree size will depend on the brand though. Can you post pics of your horse? One from the side on level ground, head about wither height, and one from a still behind him, pointed down at his withers if he’s safe to do that with. You can also take a wither tracing and photograph that too. I find it very helpful to have that wither tracing to compare to a photo of the saddle straight on the front. I can pretty much find something to fit my Arab based off of 2 or 3 pics at this point just from lots of practice. Same with the mare, but the Arab is a difficult fit and she isn’t.
Some horses will also need a wide twist. Arab boy is a medium and he cannot use a narrow twist. He bucks violently if asked to canter in anything with a narrow twist.