Well, it depends on the type of exercise sheet. The type I own attached to the girth straps, and then goes over her quarters.
I've seen some which go under the saddle, attach to the back D rings and wrap around ones.
I'd suggest you explore what type of sheet you'd like to purchase first, then (if there are no fitting guidelines) measure from the point of attachment to the back of her leg.
However, I don't think I've seen any sheets which specify their sizes in inches- just pony/cob/full.
If you aren't sure, ask the shopkeeper for fitting advice and ask them if they take returns. When you do try it on for the first time, put a sheet over your mare to keep the sheet clean in case you do need to return it.
Good luck :)
21 March 2010 10:31