I have the same eight gallon kettle that I've had since I started, about five years ago.
I brew five-gallon, all-grain batches now, using a three vessel setup, but started out with extract, and went through a BIAB phase, as well, but now I can't remember if it was a true BIAB process, or if I used a ten gallon Igloo. Anyway. Have always done five gallon batches.
My kettle is a little different than what I think a lot of people would recommend, in that it's actually wider in diameter than it is deep. Not great, but functional. I'll draw off just over seven gallons of wort (just enough to cover the handle rivets), and I usually wind up with a little over five gallons in the fermenter, once I pitch my starter.
This setup will boil over if you don't pay attention, but isn't that hard to monitor. I've only had three or four boil overs in total.
Soooooo... I would say that eight gallons is your minimum! I would definitely look for a ten gallon that fits in your budget, whatever that may be. And try to find a pot that's deeper than it is wide. Big fan of Spike Brewing here; I plan to replace my kettle with one of theirs sometime in the next year or so. If you think that splitting batches, or really huge beers might be in your future... I'd consider 15 gallons.