Postage Price Calculator for US
Think about it. You have to purchase the little envelope and the stamp to mail the standard RSVP card. A postcard eliminates the envelope, the extra weight being added from this and saves on postage!
According to the USPS:
• Maximum size is 6” long X 4 1/4” high
• Minimum size is 5” long X 3 1/2” high
I personally would suggest using the larger size so when it is sent back to you it wont get accidentally pitched with other junk flyer.
You can do some pretty amazing things on a postcard like a picture of the couple on the front, using vintage fonts and so on. I designed ours and then...my computer crashed (sigh) So I can not post it, however I have foud some great examples via google
This is of course a STD Postcard (another great idea) but it still gives you an idea on design
Here are some postcard templates free to use