I successfully smoked and cooked a 12 lb. turkey entirely in my MasterBuilt electric box smoker.  The turkey was spatchcocked and then marinaded overnight in the refrigerator in 1 recipe of the marinade for Apple Smoked Cornish Game Hens.  The next day I rinsed it off, dried it with paper towels and a fan and then preheated the smoker to 275 degrees.  The turkey barely fit on one of the shelves, so 12 lbs. is about the limit.  It was done in about 3 hours, which was quicker than I expected.  Towards the end, I reduced the temperature to 250 to slow the process down.