The USPS has specific size and weight requirements for each type of mail, from postcards to large packages.
Postcards (rectangular cardstock not contained in an envelope) have a minimum size requirement of 5 inches in length and 3.5 inches in height and a maximum of 6 inches in length and 4.25 inches in height.
Envelopes (small rectangular mail pieces no thicker than 0.25 inch) must be between 5 and 11.5 inches in length and 3.5 and 6.125 inches in height to qualify for standard letter rates [source: United States Postal Service]. There is also a " large envelopes" category — aka "flats" — with a minimum size of 11.5 inches in length and 6.125 in height and a maximum size of 15 inches in length and 12 in height.
Each type of mail must also fall within specific weight limits to be mailed at the standard rates. For example, you can't write a 12-page letter and mail it at the standard letter rate even if it does fit into an envelope that meets the size requirements. It has to weigh 1 ounce or less — the accepted limit for a letter.
You can mail heavier and larger letters or postcards — you'll just have to pay more. For example, the 2020 rate for a standard 1-ounce letter is 55 cents. But if your letter weighs 3 ounces, you'll have to cough up an additional 30 cents, 15 cents for each additional ounce. The USPS offers a postal calculator to help you figure out your costs from home.
Ultimately, there are limitations to what you can mail. The largest package you can mail (retail ground) may measure 130 inches in combined length and girth but can't weigh more than 70 pounds [source: United States Postal Service].