Any envelope larger than the maximum dimensions for a United States postal letter (6 1/8 inches high, 11 1/2 inches long and 1/4 inch thick) requires extra postage. An envelope that doesn't exceed these dimensions and weighs one ounce or less will require only a single First Class postage stamp.
What the post office calls a “large envelope” exceeds one of these letter size dimensions. Popular large envelopes used for mailing documents are 9 inches by 12 inches and 10 inches by 13 inches. These require two First Class postage stamps for the first ounce and additional postage for each ounce beyond one.
Large envelopes that are thicker than 3/4 of one inch will require package rate postage. In such cases a Priority Flat Rate envelope may be a good choice for mailing.