I would go 11' as a minimum. My ceiling is 10' 5" and I only have about 3" clearance to my garage door opener. There is 4" clearance between the lower car and the bottom of the lift. Also remember that the hinge that attaches the opener to the garage door extends down below the level of the opener. The garage needs to be deep enought that this hinge does not hit the window of your car when it's raised. See Fast82930's photo with the door raised. If you are going to have a manual door 10' is acceptable. However, if automatic, then take this into account.
Also, the lift needs to rest on one of the mechanical stops on each of the four legs. It should not just be supported by the cables. These stops are placed something like 4" apart so you may not be able to stop it where you like and must instead go to the next higher stop. This will eat into the available space you thought you had. Another point; when you get ready to lower the car, you need to raise it about 2" off the stop for the locks to clear the mechanical stops before you can start lowering it.
Another reason to go 11' or more is it will allow you to use a jack on the lower car and work on it in place, to more easily place a car cover on the lower car, and to get out without cracking your head every time. (Takes about 3 knots to remember to duck)
