To size, think about length both down the spine and the drop on each side. Quarter sheets are designed to keep horses sides and butts warm while exercising, so you want it to come to just the tail head and just to their belly for lengths. I own a “one size” quarter sheet, it fits obviously small horses, but have found max size 17hands of an athletic built horse, not narrow but not wide. If your horse is wide, like a big built warmblood type, probably see if they come in an oversize or warmblood size or make your own. They are pretty easy to design and could be made easily from two pieces of fleece. The SmartPak one you have linked is styled the same as mine, it has a cut out to fit around your saddle with velcro over the withers. This is nice. I always put mine under my saddle. Yes, you could velcro it over your own legs, but then the front flaps in the wind and it becomes like a weird cape and doesn’t keep your horse’s butt warm.
Also as a small note, I had one horse who got itchy with fleece (as a human, some fleece makes me itchy too) so I always wash new fleece items a few times to get the coating off it. Just don’t dry it in the dryer or use fabric softener. Since it is fleece, some static electricity may build up rubbing on the hair, but a quick spray of show sheen or similar product along the horse side of the butt portion works like static guard to help prevent it.
I like my quarter sheet, I got it on an online close out and it was been a good investment. I ride with it on for flatwork and jumping, whenever the temps are cold enough that horses wear their blankets.