Hmm, whats that in A4? (in Europe here, we do things the weird way!)
boatbird, like qazicat said, the A4 is bigger. 8.5x11 inches is the commonly called 'Letter size', in cm it is 21.5 x 27.9
The A4 is actually half centimeter smaller in width, but it is longer: 21 x 29.7cm
And I amost forget to answer your topic question
: for the back of my cards lately I am using a design matching the front of it. I add the basic info like name, date, card name, but complementing the front design, so each one of my card backs is different. I will scan some soon and post then so you can see them.