Letter envelopes come in a numbered series, and the numbers do not obviously map to sizes until you have the chart in front of you. Here is the full run.

The commercial (letter) envelope series

#6 3/4: 3.625 x 6.5 inches, for checks and small statements
#7: 3.75 x 6.75 inches
#7 3/4 (Monarch): 3.875 x 7.5 inches, often for personal stationery
#9: 3.875 x 8.875 inches, the classic reply envelope that fits inside a #10
#10: 4.125 x 9.5 inches, the standard business envelope
#11: 4.5 x 10.375 inches
#12: 4.75 x 11 inches
#14: 5 x 11.5 inches, for thicker contents

Which one to use

The #10 is the default for a business letter, holding an 8.5 x 11 inch sheet folded in thirds. Step up to a #11 or #14 when you are mailing more pages or a small booklet that a #10 would bulge against. Drop to a #9 for a return envelope that tucks inside a #10 mailing.

Window or plain

Many of these come in a window version, where a die-cut window shows the address printed on the document inside, so you do not address the envelope separately. That is why billing and statement mail so often uses #9 and #10 windows.

Flat items are different

This whole series is for folded letters. If you are mailing something that must stay flat, a photo, a certificate, an unfolded document, an envelope is the wrong call; a rigid mailer keeps it from bending. For folded correspondence, pick from the numbered series above and match the number to how much you are sending.