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 in height.
Letters (small rectangular mail pieces no thicker than .25 inch) must be between 5 inches and 11.5 inches in length and 3.5 and 6.125 in height to qualify for standard letter rates. There is also a "larger letter" category with minimum sizes of 11.5 inches in length and 6.125 inches 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 current rate for a standard 1-ounce letter is $.41. But if your letter weighs 3 ounces, you'll have to cough up an additional $.37.
Ultimately, there are limitations to what you can mail. The largest package you can mail must be less than 130 inches on its longest side for Parcel Post, and it can't weigh more than 70 pounds.