Buying a card for someone is no small feat. First, you have to remember it's their birthday. Second, you're really busy. They should acknowledge that, and thank you for your thoughtfulness.
The large card is for them, the mini card is for you so that they can finally give you the thanks you deserve. Just make sure to write your name on the mini card envelope so they don't forget who is their awesome friend.
Size: 4 1/4" by 5 1/2"