Here's the sizes of common envelopes and their equivalent sizes in letter ratings

A-2 = 4 3/8 x 5 3/4"
A-6 = 4 3/4 x 6 ½"
A-7 = 5 1/4 x 7 1/4"
A-8 = 5 ½ x 8 1/8"
A-10 = 6 x 9 ½"