A customer of mine, who is getting married in Las Vegas, had the idea to make their save the date announcement look somewhat like a poker chip. This meant we would surely need something circular.
Available are circular cards or circular magnets, 4 inches in diameter. The minimum order of magnets is 100; The minimum for cards is 20.
Check out her design:

If you went with a card instead of a magnet, you would have a white back, with a printed front.
This design idea can easily be modified a bit for another wedding related event- such as a bachelor party- or just a personal casino themed party invitation.

If mailing these out alone, you would probably want to go with a 5.7x4.35 inch envelope. It's a common size, and thus less expensive than other envelope sizes.