Ewww!! Bad bad bad!!!!
I haven't ridden in that exact saddle, but you can tell it's one of those generic, cardboard, painted-on-color, cheap, made in India, off-balanced, pieces of garbage!
Unless you ride in it like...once a year...you'll be disappointed. (And you'll still be disappointed for that one day and wonder why you wasted the $150.)
As for the fittings that come with the saddle. I bet the irons and bit are not Stainless Steel, so they'll rust. The Stirrup Leathers and Bridle are probably stiff and cheap; could possibly break if under stress. Also, they won't be dyed properly and the color will come off when you (try to) oil it or clean it. It has a low budget web girth with no elastic (for ease of tightening and comfort of your horse).
Just bad news all around.
Sorry, but I'm really anti-crappy saddles.
As for figuring out saddle size...a saddle is measured from the little metal pin on the pommel (front) to the center of the cantle (rear edge of seat). Kind of a diagonal line. Hold the tape measure tight, don't curve it with the seat. When seated, you should have about 4 fingers width behind your rear and the edge of the cantle. Kid's sizes are generally 13-15.75 inches and small adult in a 16 or 16.5, and adults in 17-18.
Also, horse's size (tree size) is just as important as (if not more than) the seat size. That saddle you listed only comes in one tree size (Wide) which obviously won't fit all horses!