What is the largest size postcard that can mail at USPS postcard rates and not letter rates?

Postcard postage rates are cheaper than letter rates, but only up to a certain size. Go bigger and the piece mails at the higher letter rate instead.

• The largest postcard that qualifies for reduced postcard rates is 4.25" x 6".
• Anything larger than that mails at letter rates, not postcard rates.
• Check the USPS guidelines for the full size and rate breakdown.