Well now,that depends on how you want to do it,and how you want it to appear.
Chefs will typically select about 12-14 lb birds,because they cook evenly,and well.
It is diffiult to cook large birds,even in a commercial oven.
I had a Smokette,008,which is the shorter version of the 55.
I used to smoke anything,and always tried to exceed the limits on size and weight.
If someone is dumb enough to want to prove it can be done,it can be forced to waller around about 25 lbs +/-.
Don't ask how I know.
If you read the Turkey 101,it should guide you that you can section up a large bird.
The backbone,wingtips,etc can go to your stockpot.
The leg qrtrs[dark meat] can go in earlier than the breast,and add the breast later-to pull both at the same time.
If you wish to present whole at the table,you might get a pair of 12 pounders?
Hope this helps a little.