What size kettle...

is ideal for boiling 6.5 gallons of wort? What I mean is, I suppose there is sort of an optimum size. Two big and it's too cumbersome to lift, won't fit right on burner stand, etc. Too small and you get too many boilovers. So, I guess what I'm saying is what is the smallest size that is "comfortable" to fit a full boil in it?

My currentl kettle is the aluminum one that came with my turkey fryer kit. It's not really too bad a pot except that it only just barely holds the whole boil. I don' know if I can get more than 6 gallons in it. (Part of the problem is that the rivets on the handle on one side leak, so if I fill it up above them wort runs down the outside onto the burner).

There seem to be some 9 gallon SS kettles around that would fit well on my burner. Would that be a good size?

A 9 gallon pot should be just fine. Another variable to think about is the dimensions of the pot. I think a tall narrow pot has a tendency to boil over quicker that a pot of the same volume that is shorter and wider.

I have an 11gal pot and its perfect. I never worry about boil overs so the boil is pretty much set and forget. I would say that that about 11 should be the minimum gives the ability to do some high gravity bress as well.