Seems to me I have cooked 17-18 lb turkeys on the WSM, but I'm not sure I have gone larger.
But here's a simple trick. If you have a huge bird, get it spatcocked, which means it's butterflied. I buy turkeys from a store with a meat counter and they will cut the turkey for me. It's also possible to get a frozen turky cut in half on their bandsaw. So you can save a bit of time and effort if you have a market that will cut them for you.
But if not, you can thaw the bird and cut it yourself. I bought a big pruning shear from a garden center for cutting turkeys. (I don't use this in the garden. It sits in my kitchen and is only used for food. In fact, it's only purpose is cutting turkeys.)
Anyway, this gives you two flat halves. They are a lot easier to store in the fridge than a whole turkey, and they can be brined by putting each half into a jumbo zip lock bag with the brine. It beats the heck out of trying to put a 5 gallon bucket in a fridge with a turkey bobbing in it and a plate on top to hold the turkey under the brine. Just cut the bird in half, put them in the 2 gal size ziplocks, add the brine, stack them in a pan in the fridge, and when you cook them, put one on the bottom rack and one on the top rack. Simple.