PO Box sizes 100, 102, 103, 104, 105 What Sizes Are Available? Across the U.S. Postal Service, PO Boxes are available in 5 sizes. However, not all Post Offices have every size. Size 1 - X-Small (3" x 5 1/2" x 14 3/4") Our smallest box fits 10-15 letter-sized ...
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