This is a question that we are asked a lot.  For an easy answer, I draw on my experience as an avid boot-er with my own personal herd, as well as our total sales. 

We sell about 85% medium boots.  The rest breaks down to about 10% larges and 5% smalls.  Medium is by far our most popular size.  I am sure that could change a lot if you were to look at a different area of the equine industry.  English horses would run larger, Arabians smaller, and so on.  But for our customers with a majority of western performance horses, that's our breakdown.

Personally, I have one monster big head horse that is right at 16 hands and heavy boned that needs a large, and one young reiner (3 years old, 14.2, and a little on the fine boned side) that is borderline between a small and medium.  But all the rest of my 6 riding-age horses get along just fine in a medium. 

Isn't that funny? My herd of 8, 7 QH and one paint, runs almost exactly the same percentages per size as our average sales here at the store!

I do boot my horses, front boots and either hind or skids, plus bell boots depending on the ground and the horse's stride, almost every time I ride.  To me, it's just a little insurance.  If I prevent a horse from going to the vet one time, I've paid for his set of boots.  And that doesn't even touch on the time I save in training and being able to use my horses when I want.  Not missing riding or training if they are injured or even just sore.  Another thing to watch, most horses will use the same size bell boot as splint or sports medicine boot.  If you are having to move up a size on your bell boots, you might want to make sure that you have enough overlap in your other boots.