The "standard" envelope depends on what you are mailing. For business letters in the US, one size dominates, but invitations, cards, and documents each have their own.

The standard business envelope: #10

The #10 envelope is the default for business letters in the US. It measures 4.125 x 9.5 inches and holds a standard 8.5 x 11 inch letter folded in thirds. If someone says "standard envelope" in an office, they almost always mean a #10.

Other common sizes

#9: 3.875 x 8.875 inches, fits inside a #10 (used for reply envelopes)
#6 3/4: 3.625 x 6.5 inches, for small statements and checks
A7: 5.25 x 7.25 inches, the go-to for 5x7 invitations and greeting cards
A6: 4.75 x 6.5 inches, for smaller cards (more on these in our A6 envelope guide)
6 x 9 and 9 x 12: catalog and document envelopes that take pages unfolded

How to choose

Work back from what goes inside. A folded letter wants a #10; a flat 5x7 card wants an A7; full-size documents that should not be folded want a 9 x 12. Leave a little clearance so the contents slide in without forcing.

Printing your own

Custom envelopes carry a logo and return address and match a brand's other stationery. If you are mailing flat items that must not bend, an envelope is the wrong tool: a rigid mailer keeps prints and certificates flat instead. For documents and letters, a printed #10 is the workhorse.

In short: the standard US business envelope is the #10 at 4.125 x 9.5 inches. Match the size to the contents and you will not end up folding something that should have stayed flat.