As a general rule of thumb, most small-sized definitive stamps are about 25-26mm high. Most standard-sized horizontal commemoratives are also about that height. Most standard-sized vertical commemoratives are 40-41 or so mm high. I like to use this handy perforation gauge ($2.39 delivered on
ebay) which contains a mount sizing gauge on the back. That elimates the guesswork as to which size mount is necessary.
http://www.ebay.com/itm/Showgard-60...AMXQHDlRbsaw