There is a standard in western saddles. For the horse, there is semi quarter horse, quarter horse, full quarter horse etc. Even those basic standards can vary by manufacturer. Its best to have the horse first, then buy a saddle that fits it. Its easy to fit yourself to a saddle but its extremely difficult to get a good fit for your horse.
Depending on how much you want to spend for a good fit, you can estimate the size. Or you can spend the big bucks and have a custom saddle made. Even then there is no guarantee since a horse will change shape many times throughout its life.
Here is a website that has been very helpful to me Western Saddle Fitting and Different Tree Sizes