IMHO, A 9 gallon kettle is plenty. The one I currently use is 9 gallons and I've never had any issues with it.
Figure this way, say your shooting for say 5.5 gallons in the fermenter. So theoretically your system boils a gallon off in an hour (this varies) and you need to boil for 90 minutes. You would start with 7 gallons in the kettle. This still would leave you with 2 gallons of head space. This IMO is a worse case scenario and you still have plenty of room.
hope this helps