I've done whole turkey breasts in my 18.5" WSM but never a whole turkey.
But you could probably cook as big a turkey as you'd normally find in a grocery store.
An 18.5" WSM measures about 17" across and about 17" from bottom grate to top of lid. That's a large space.
A quick search finds that a 34qt Turkey Deep Fryer measures about 13"x17" and will hold a 25lb turkey.