We are serving 15 so what size do we need?

Question

But how do I cut the turkey?

Answer 1

A whole turkey yields about 1/2 it's weight in edible meat. You should plan on a pound of meat per person, which would yield 8 oz of meat per person.
This site will show ( with pictures ) you how to carve a turkey.
http://www.occis.com/cis/holidays/thanksgiving/Carving/carving.html

Answer 2

You should get a 20 pound turkey or thereabouts.