I have them - can't really say if they work or not to be honest but they look lovely and comfy for my old boy and they've lasted into their second year.
I'd go for full size for the hind legs - They are fab - I've got a pair -infact I need another pair! I've had them for about 6years now and they've seen me through 2 different horses with 2 sets of different issues - Used them with great success on an arthritic cob and then currently using them on H to help her circulation overnight.
Can't recommend them highly enough!
I used to have a set a few years ago for an older horse that had really bad windgalls and was a bit stiff. They fit really snuggly and provide a lot of support. He was much better with them on. Not sure whether the magnetic pieces had an effect, but from what people have been saying about the headpieces, I wouldn't be surprised!
I used to put them on the back legs at night and front legs in the day.
Unfortuantely I sold mine, so have just ordered a new set. I have splashed out and got front and back ones for just under £150 including postage from http://www.whiterose-equestrian.co.uk/.
I had a large for my TBx and he shredded them fairly quickly. He does suffer windgalls when he stands in too long but they didn't appear to make any difference really so I've not replaced them.