I have a Corriente that's FQHB and a 7 inch gullet and that thing fits everything. I've had it on a 13 hand Shetland x QH and I'm currently using it on my 16.1 hand Belgian x QH. Fits both of them great.
Many folks don't consider both measurements on the front end of the saddle. There is the gullet width and there is also the angle of the bars. Not all horses will fit into one of the mass-marketed saddle tree sizes as they may need flatter bars with a narrower gullet or they may need more angled bars with a wider gullet, it all just depends on how the horse is built.
I second the suggestion of the fitting forms. That way, you will know exactly what fits and what doesn't.