I wanted to try a 10 Gallon BIAB but I only have a 13.5gallon Keggle, problem! I figured it was going to be a PITA but I'd give it a shot. I used this recipe, http://www.homebrewtalk.com/f66/centennial-blonde-simple-4-all-grain-5-10-gall-42841/
And this is the way I did it.
I added 11 gallons and heated to 156 then added 18.25 lbs grain (I used an extra lb of 2row) ended up with 150, perfect! Mashed for 60mins added heat once. I lifted the grain bag out slowly and put it in a bucket and squeezed, I ended up with 10gal that I brought to a boil, standing by with a squirting bottle for the hot break to control boilover. 20 mins into the boil I added 1 gal of boiling water from my electric kettle and added another gallon at 40 mins. After the 60 min boil I ended up with about 10.5gallon so I added another gallon and chilled. I ended up with an OG of 1.042, pretty good! 20 lbs of grain is about as much as I would feel comfortable with, at least I know that I can do it!