Update (3/01/2015): I apologize for not including the following information upon first posting this card. It was on my blog, but I neglected to include it here.
Stampin' Up! has a stamp set called B.Y.O.P. (Buy Your Own Present). It's a great set to use if you're giving a gift card or cash as a gift. I found the measurements at My Little Craft Blog. She does a great job of breaking down each of the measurements.