For details, see the USPS website.
To qualify for mailing at the First-Class Mail postcard price, it must be: Rectangular, At least 3-1/2 inches high x 5 inches long x 0.007 inch thick, and no more than 4-1/4 inches high x 6 inches long x 0.016 inches thick. As a guide, most index cards are thick enough. Make sure your mailpiece meets the minimum thickness requirement. Thin, flimsy pieces tend to get caught in mail processing equipment. If your mailpiece gets damaged in the equipment, then your message doesn’t reach your voters.
For standard postcards: Must be rectangular. Maximum size: 6 inches long by 4-1/4 inches high by .016 inch thick. These cost $0.36 to mail (postcard stamp)
For large postcards: Maximum size: 11-1/2 inches long by 6-1/8 inches high by 1/4 inch thick. These cost $0.55 to mail (regular first class mail)
You can mail odd-shaped cards, but we suggest you confirm postage requirements with the post office before investing a lot of time in creating and addressing these.