I think you'll be fine if you are just using it for a BK. Bigger would be better to allow for potential boilover room.
However, if you plan to use it as a mash tun with a water to grain ratio of 1.5:1. a general rule of thumb for mash tun capacity is 2lbs of grain for every gallon of kettle volume. So your 9 gallon kettle would comfortably fit 18lbs of grain with 6.75 gal of water.