Here is a list of common size preconverted envelopes. There are many other sizes that you can get converted that are special sizes but are not considered stock sizes.
Commercial Envelopes
6 1/4-3.5×6
6 3/4-3.625×6.5
7-3.75×6.75
7 3/4-3.825×7.5
Monarch-3.825×7.5
Check-3.625×8.625
9-3.825×8.825
10-4.125×9.5
11-4.5×10.375
12-4.75×11
14-5×11.5
Text Envelopes(Usually w/Square Flap)
A-2 4.375×5.75
A-6 4.75×6.5
A-7 5.25×7.25
A-8 5.5×8.125
A-10 6×9.5
Baronial Envelopes(Pointed Flaps)
4-3.625×5.125
5-4.125×5.5
5 1/2-4.375×5.75
6-4.75×6.5
110-5×7.25
Catalog Envelopes(Open End)
6×9
6.5×9.5
9×12
9.5×12.5
10×13
Booklet Envelopes(Open Side)
5.75×8.875
6×9
6×9.5
6.5×9.5
7.5×10.5
9×12
10×13
Remember: Your printed product finished size should be smaller than the actual size of the envelope so that it will fit easily into the envelope. We like to have at least an 1/8″ on each side.
Source by Printing Guy