I went with an 11 gallon (44 qt) bayou classic kettle
. I started with extract and then went to BIAB about 6 months ago. I am very happy with it. Even with that much extra room I have still had a few boil overs. But I am not very vigilant with my stirring and heat control.
They always happen when I am bringing it up to the initial boil. My recommendation would be to go bigger than 9 gallons.
You do need some extra room for the mash, but I would say that you are still more driven by the boil over risk when using a smaller kettle.
I bought a ball valve and weldless bulkhead a while back, but I have yet to install it. I am undecided on what I want to do about a false bottom, or mesh hose, etc. I just ask the SWMBO to help me dump it in the carboy.