Sizes for first class mail

The sizes for First Class Mail in the United States are as follows:

Letters

• Maximum size: 11.5 inches (29.2 cm) long, 6.125 inches (15.6 cm) high, and 0.25 inches (0.64 cm) thick
• Minimum size: 5 inches (12.7 cm) long, 3.5 inches (8.9 cm) high, and 0.007 inches (0.18 mm) thick

Large Envelopes (Flats)

• Maximum size: 13 inches (33 cm) long, 15.5 inches (39.4 cm) high, and 0.75 inches (1.9 cm) thick
• Minimum size: 5 inches (12.7 cm) long, 3.5 inches (8.9 cm) high, and 0.007 inches (0.18 mm) thick

Packages

• Maximum size: 13 inches (33 cm) long, 18 inches (45.7 cm) high, and 13 inches (33 cm) wide
• Minimum size: 3 inches (7.6 cm) long, 2 inches (5.1 cm) high, and 1 inch (2.5 cm) wide

Oversized Packages

• Maximum size: 24 inches (61 cm) long, 30 inches (76.2 cm) high, and 24 inches (61 cm) wide
• Minimum size: 3 inches (7.6 cm) long, 2 inches (5.1 cm) high, and 1 inch (2.5 cm) wide

Note: These sizes are for standard First Class Mail rates. Larger packages may require additional postage or special handling.

It's also important to note that the USPS has specific guidelines for preparing packages for mailing, including requirements for labeling, packaging, and handling. You can find more information on the USPS website or by visiting your local post office.