I recently came across this video that gives some excellent information about saddle fit and the elements that need to be considered.
Please note that I do not endorse any particular saddle or saddle fitter with this blog - but I do want to raise awareness.
Also - Jochen mentions that the gullet needs be wide enough for the horse, which is completely true. However be careful of gullets that are too wide as they can be destabilizing.
Saddle fit is both art and science and my best advice is to spend time with a dedicated and well trained saddle fitter.
Here are some excellent supporting articles.