Question: What’s the smallest envelope permitted through the post office? Answer: 3-1/2-x-5 (inches) is the smallest legal mailing size for an envelope in the US. The smallest “mailable” size offered on our site is 4-bar Envelopes / A-1 Envelopes. If you are browsing through our mini envelopes, please note they are not intended for mailing. To find more info about mailable size limits for envelopes, jump over to the post office site and view more detailed explanation.