Right. The mm refers to the radius, so the smaller the radius, the smaller the bit coming off the corner of the card. If you look at regular playing cards and most tarot decks, you'll see a small radius arc. Anyway, should it turn out that you'd like a larger snip off the corner, you'd have the option to go back with a larger radius cutter.
I wish I'd had a choice, mine is a bit rounder than I'd like, but it's the only one I could find. Go for the little 'un, your cards will look more like they did out of the box.
Yes! Just as Debra said. When I bought mine, there was a choice of three different sizes, so, not knowing which would work best, I bought all three and tested them out on playing cards. Smaller is definitely better!