get a grain bag and do partial mashes. you can mash 5+lbs of grain in a 6 gallon pot. i am currently doing 8 in a 7.5 gallon and i have gone as high as 11.
partial mash is really where its at imo. you don't need much equipment. all the benefits of AG (basically at least), but less hassle. biab with 10+ lbs is a pita.
to answer your question though, don't need a bigger pot if you don't mind adding your sparge water half way through the boil. a mash tun, and another pot that you can heat sparge water in, or something else to catch your runnings while you heat your sparg water in the BK.