Based on US Postal regulations, here are some tips and visual guides on how to design your postcard for mailing:

The smallest mailable postcard is 3.5x5.
The largest postcard that can be mailed at the postcard rate (currently 27¢) is 4.25x6.
Any postcards larger than 4.25x6 require first class postage (currently 42¢)
SQUARE postcards require additional postage of 13¢
TIP: If you mail square postcards in a rectangular envelope, you pay just 39¢ and avoid the 13¢ surcharge NM indicates Not Mailable