If you're mailing a printed check, the right envelope depends on whether it's folded and which check format you're using — getting this wrong is a common reason a check doesn't fit cleanly or needs an awkward fold.
For standard business (voucher) checks
A business check at 8.5 x 3.5 inches, folded once (in thirds, like a standard letter), fits a standard #10 business envelope (4.125 x 9.5 inches) — the most common envelope size for this purpose, and often sold specifically as "check envelopes" with a window cut-out aligned to the payee line.
For personal checks
Personal checks are smaller, so they typically fit a standard #6¾ envelope (3.625 x 6.5 inches) folded once, or a #10 envelope without folding at all if you prefer not to crease the check.
Window envelopes are worth using if your check layout places the payee's name and address in a consistent spot — it saves hand-addressing every envelope and reduces the chance of a mismatch between the envelope address and the check itself.