General rule of thumb is a BK that has twice the batch volume.
Example: 20 Gallon SS BK for 10 gallon batches.
However, the higher the gravity (e.g., 1.090 versus 1.050), and the larger the pre-boil volume, the greater the likelihood of a boil-over.
It won't boil over (well, usually) if you keep the lid off while the flame is on. Having the lid partially on is fine (usually) if you're trying to sanitize the lid before Knockout--just rotate the lid every 5 minutes, but leave plenty of room for the steam to escape.
FermCap-S is good insurance, too.