Our Thanksgiving turkey last year was a Rio Grande, which is basically pretty close to the wild type turkeys found in the western United States. At 8 months old, her dressed weight was just over 10 pounds, and she was free-range. Tasty, but fairly dry and not too meaty. She served six people pretty comfortably with no leftovers--she was mostly bone, but we boiled those to make some yummy broth. This year we are going with Royal Palm (if we can get them to hatch...no luck so far) and they should be between 6 and 8 months old by Thanksgiving, we expect they'll be in the 10-12 pound range as we will keep them penned. Heritage turkeys don't always have a lot of breast meat to them, I have found, so if that is really important to you, a broad-breasted type would be better.
My advice would be to get a few, and that way you can serve more than one if needed.