Ok, basically as long as your horse is a pretty regular shape any size will fit, as long as it has good clearance above the wither (2 -4 fingers width) Doesn't sit on the top of the shoulder & restrict movement & sits well on the back, meaning it doesn't look too short or too long & also that it 'fits' to the horse & doesn't just seem to be plonked there (sits too high). As this will result in uneven weight distribution
You can't really get a correct measurement once the saddle is made, but as a basic figure, Semi bars will measure around the 5.5" mark, under that & it will most likely have regular bars, above that full bars. If the measurement is over the 7" mark then it is an extra wide make. For a bare tree you would take .5 to 1” of those measurements, sometimes more, sometimes less.
But really it's the height and the width combined that determines the best fit, if the gullet is too short it will sit on the wither & rub, while if it’s to wide it will sit too low on the shoulder & also rub on the wither.
Below is a pic I took of where to measure, A to B is the bar width & determines if it is regular, semi, full or extra. & C to D is height.
The angle of the bars & the rise of the pommel determines these measurements, but as I said if the saddle is already finished, your measurements won’t be exact.
Another thing to remember is that the bars don’t only determine the width, but also how the saddle sits, this all comes down to the rocker (or the curve), twist & flare of the bars.
Also good to remember that the name of the saddle in no way determines what bars are used, it only represents the fork on the tree.
Most of our saddles are have semi bars, we find that these fit the majority of our horses well. From our heavy set QH’s, to my daughters finer set Q pony & even our Standy mare. Just make sure you buy a well made, quality saddle. As most good saddle makers aim for a general fit, rather than an exact fit, unless it is custom ordered.