I think you could get by just fine with a smaller one if you aren't planning on having a bunch of leftovers. Assuming this is a regular meal(not a Thanksgiving feast), a 16 - 18 pound bird would probably be enough. We generally have 20-24 people for Thanksgiving, and with all of the other side dishes, a smaller bird is plenty. My experience is that the guides tend to overestimate by quite a bit. When I was buying deli meat for about 30 people, the deli person told me to figure 1/3 pound per person. That would have been 10 pounds! I went with 6 pounds, and still had way too much, because there were other things to eat besides just sandwiches.
BTW, have a great time at your reunion! What fun!
Less rhetoric, more cowbell!