measured mine. its 6 1/4". the tabs are set at about 1/2". so about 1/2" goes inside the kettle. i've seen many home made ones taller. you can also put in some bolts so you can lay in another rack and use it as a smoker.
Making it a little taller with the rotisserie skewer hole at the same height off the coals wouldn't be a bad idea. I did a large turkey (16-18#'s) last year and it barely cleared the lid after a lot of trussing. The motor handled the weight just fine.