I was always taught that you need to be able to slip your hand in between the tree and their shoulder for it to fit properly. Also, you should have something like three fingers-width of clearance between their withers and the pommel of the saddle with you in the saddle.
Best way I've found to tell if ANY saddle fits is to lunge them up to a sweat while wearing it and check their sweat pattern. No dry spots=good fit. Dry spots=run away screaming. Obviously, if you're buying new, this isn't an option, but I've never bought a new saddle in my life, so...lol
Also, some horses will let you know right away if they don't like the way the saddle fits. My old gelding was good about that. I put a cordura saddle on him once and he was a complete jerk with it on (luckily I only lunged him in it, didn't try to ride). When I was lunging him, he was bucking AT me (which he knew was an absolute no-no) and kept darting out to the end of the line, trying to get away. When I got back up to the barn, I told my friend "This saddle is a definite NO!" When she asked why, I told her what had happened. When we took the saddle off, sure enough, there were two dry patches on his shoulders. That being said, other horses aren't as picky about saddle fit, so it might be harder to tell just from their behavior.
If you're buying new, the best idea is to get a withers tracing of your horse. Most companies will let you send in the withers tracing and they'll match it to the size tree your need. Or, if you're buying from your local tack store, take the withers tracing with you (on a piece of cardboard) and check it against various saddles you're interested in.
So are you suppose to be able to slip your hand in through the shoulder area when tightened or loose?
I never had a brand new saddle..new to me but all of them have been used. I plan on getting a used one..I don't like new saddles...old ones are already worn in.
Gidget lets me know,too. She let me know on my mom's barrel saddle. She will act a fool and will try to bite..switch too my saddles and she is fine.
I used the same concept for mine. I knew the saddle I have didn't fit but I didn't know by how much (it's SQHB with like a 5" gullet). With the template from the first website, the wide template fit Abby perfectly (So she would need FQHB) and then with the gullet measurement, she would need around 7".
I apparently have a very tolerant horse because I've been using that saddle for a year and she just started getting a sore back from it and became a jerk recently.
Well I was told that a 7" gullet usually ends up being FQHB....so that's correct,right? I have been using a billy cook FQHB and it seems to fit her well so do some saddles differ from others because usually she is a SQ.