I don't think they sell any slices -- you order by the half-pie, which is 3 slices, which is good because you get 2 corner slices. (The square pies are 6 slices total.) You can also share a pie with someone and get different toppings on each half. I usually could eat 2-2.5 slices for lunch depending on the heaviness of the toppings.
They do takeout, eat in and sit down. The dining room is a bit of an odd time warp -- they're on the second floor of a brownstone sort of building so you go down a hallway and enter there.