To match our holiday cards, I made Gift Tags for the girls... Last year the girls were having a hard time finding out who was the gift for, and even though my oldest can read all three names, my other two daughters cant... So, each one is 2 inches by 2 inches card, printed in cardstock... Now, they are using them as play cards.
As for the gifts for other people, we made folded cards. Here are the front of the cards, and the back is simply one of the solid colors (in all of them the color that is used around each kid's picture. They are 3.5 by 2 inches when closed (business card size).