Postcard

According to the Postal Service, postcards should be:

Dimension	Minimum	Maximum
Height	3-1/2 inches	4-1/4 inches
Length	5.5 inches	6 inches
Thickness	0.007 inch	0.016 inch

Any item smaller than the minimum dimensions is not mailable.
For postcards, length is the dimension parallel to the address.
Postcards must be rectangular and be made of unfolded and uncreased paper or card stock.

The 3.5x5.5 inch height is perfect for fitting into the pocketfold and four can be cut from 8.5x11 cardstock with little waste. I'm guessing 60 lb. stock should be OK (no idea how heavy .007 - .016 inch equals and I left my micrometer in high school).