Any envelope exceeding the maximum dimensions for a United States Postal Service letter (6 1/8 inches tall by 11 1/2 inches long by 1/4 inch thick) demands additional postage. If an envelope does not exceed these dimensions and weighs one ounce or less, only one First-Class stamp is required.
What the post office refers to as a “big envelope” surpasses at least one of these dimensions. Popular dimensions for large envelopes used for sending documents are 9 by 12 and 10 by 13 inches. The first ounce requires two First-Class postage stamps, and each succeeding ounce requires additional postage.
Large envelopes thicker than three-quarters of an inch require package rate postage. In such instances, a Priority Flat Rate envelope may be a suitable sending option.