What size tart pan should I use?

It would depend on the size of your apples. I make this tart most of the time with tiny apples -- like the size of crab apples; about 2 inches in diameter. Then I would use an 8 inch pan. If you are using a big Honey Crisp, I would use a 10-12 inch. The dough for the crust will accommodate up to a 12 inch tart pan.