First Class “Flats” The Postal Service uses the word “flats” to refer to large envelopes, newsletters and magazines. Weights and Sizes To qualify as a first class flat, envelopes must: Have one dimension that is greater than 6-1/8 inches high OR 11-1/2 inches long OR 1/4 inch thick Be no more than 12 inches high … Continue reading First Class Flats