How to measure THE HORSE for a saddle?

What am I supposed to measure? I have two saddles but they don't fit him properly. It isn't a very good picture but the horse I adopted is Mouse. Do you think he need a Arabian type saddle? a href="http://www.wildhorseandburroexpo.com/prison-mustang-program/february-2011-auction/fit" rel="nofollow"http://www.wildhorseandburroexpo.com/pri…/a Thanks.