Here's some info from the USPS:
Postcards must be rectangular to qualify for First-Class Mail® prices.
Oversized postcards will be charged as a letter or large envelope.
Minimum: 3 1/2" — Maximum: 4 1/4"
Minimum: 5" — Maximum: 6"
Minimum: 0.007" — Maximum: 0.016"
(NOTE: .016" thickness equates to Cardstock Paper Gauge of approximately 33 to 26; a common paper used is 110 pound Cardstock)
The address side of a postcard is the side bearing the destination address and postage. The address side may contain a message area.
Miscellaneous graphics such as symbols, logos, or characters that appear on the address side of cards not containing a message are generally acceptable provided the items are not intended to convey a message.
To receive the postcard price the message area on the address side must be divided vertically or horizontally and meet the following additional standards:
Vertically divided cards
The address side of the card must be divided into a right portion and a left portion, with or without a vertical rule. The left portion is the message area.
The destination address, postage, and any United States Postal Service marking or endorsement must appear in the right portion. The right portion must be at least 2-1/8 inches wide (measured from the right edge of the card, top to bottom inclusive
Horizontally divided cards
The address side of the card must be divided into an upper portion and a lower portion, with or without a horizontal rule. The portion of the address side that does not contain the destination address is the message area.
The address, postage, and any Postal Service marking or endorsement must appear within the portion containing the destination address. As an alternative, when the destination address appears within the lower portion, it is permissible for the postage, return address, and any USPS marking or endorsement to appear in the upper portion. The portion bearing the destination address must be at least 1-1/2 inches high.
http://faq.usps.com/?articleId=219323 (FAQ: What is a Postcard?)
http://faq.usps.com/?articleId=221639 (FAQ: How do I Address a Postcard?)