Each model Weber grill has a specific amount of clearance between the surface of the cooking grates and the underside of the lid.
The charts below outline the amount of space on each model, so you'll know how large of a turkey you can fit on your grill.
These measurements don't take a rotisserie into consideration.
Due to variations in the dimensions of each individual bird, it's usually best to measure the turkey that you intend to cook before you buy it if space on the grill is a concern.