Reagarding size, if your horse is 14"hh to about 15"2hh, horse size shipping boots fit. If he is taller, they make larger wraps. This is an example of good, hard-wearing shipping boot: Cashel Boomers Combo Standing Wraps - Statelinetack.com
Re: a previous post--the Velcro DOES wear out. I've saved the oldest (1986 vintage) shipping boots bc they've been washed so much that they are baby soft. I used those sometimes as a substitue for quilts for long-distance hauling, but I use polos on top of them bc the Velcro is way past it.
Caution: IF you horse has NEVER been wrapped you will want to teach it to him. If he's a calm guy, he'll pick his back feet up like he's walking through the mud, the very first time he has them on. If he's spooky, you'll need to wrap him with polos on the front legs first, then lead him around to get used to them. It will take several sessions to teach him to handle the front and back polos, and then he'll be ready for shipping boots and polos with quilts.
IF you go with shipping boots, the key is wrap as tightly as is possible. If you used quilts and polos, you MUST wrap 'inside to outside' or you will put pressure on the tendon, and not the bone.
My Vet taught us to wrap a long time ago when I took a Horse Health Care Class at his clinic. I've even wrapped 4 horses, with quilts and polos in a RECORD 25 minutes flat!! The total time for wrapping and loading was 35 minutes--MY personal best.