First-Class Mail is a USPS trademark. The minimum size for First-Class Mail is 5 inches long, 3-1/2 inches high and 0.007 inches thick. The maximum size is 11-1/2 inches long, 6-1/8 inches high, and .75 inches thick. The maximum weight for letters is 3.5 ounces.irst-Class Mail International is a USPS trademark. Note: The cost of postage will appear on all international shipping labels. Maximum weight for letters is 3.5 ounces. Maximum weight for large envelopes is 4 pounds.