Best shape possible. It is important to be in the best pre-operative health possible to give yourself the best chance of getting a good result. This includes losing weight. It is sometimes difficult to exercise and diet when one has spine related pain. Talk to your surgeon to layout a plan to help get you to lose weight and get in the best shape possible.
How much, how tall? Usually we have an upper limit of 350 lbs for spinal fusion. It's smart to not exceed 250 lbs to have this surgery done. Your sugery is a partnership between you and the surgeon. Do your part to ensure success- lose the weight. If you have further questions, in your area is stonybrook neurosurgery dr. Rob galler. See him for a great opinion.