I work with a printer that specializes in printing cards, and he told me that "The typical greeting card sizes are as follows:
5x7 (with an A7 envelope)
4 1/2 x 6 1/8 (A6 envelope)
4 1/4 x 5 1/2 (A2 envelope)"
We chose to go with the A2 size, as he described them. Now I know that some countries have a different designation for a paper size of A2. I get it can be confusing, and that is why we listed the actual size of the card to try and decrease the confusion. It hasn't helped. Next year, we just won't use the standard card designation...