I did just see McGuire use one I think it was last week.
#10 envelopes/standard business are 4 1/8 x 9 1/2. So since I tend to go with a quarter inch all around smaller..unless it is bulky then 1/2 inch....That would be a card that is 3 7/8 by 9 1/4. I think it depends on how much room you like to have.
So one card to the 8 1/2 x 11 piece of CS..but you will have sentiment size strips left over.