An anti-foam additive can help maximize the volume you can comfortably boil in a given kettle. I use Five Star Defoamer-105 from Austin Homebrew. I've used my 2oz bottle in about 40 batches and it's about half gone. I use four drops in a full boil, one drop in a starter, and occasionally three or four drops in the fermenter.
I boil five gallon (6.5 preboil) AG batches in a 7-gallon kettle.