I recently just made a brew that used 23 pounds of grain. I have a 10 gallon gatorade jug, and had a 1:1 water to grain ratio. It worked out just fine.
I'll be trying to max out my my capabilities and make the biggest beer possible for the next one. With a ten gallon bin and a water to grain ratio of 0.75, I think I can get 35 lbs of grain in there. I'm not sure what to expect from it... may not be pretty. Since there wont be a lot of room on the top, I'd imagine you'd have to recirculate a lot with sparge water to get a good rinse. Should be a good experiment.
There is an equation in Palmer's "How to Brew" that calculates the volume of your mash.
Mash Volume (qt) = grain (lb) * (water to grain ratio + 0.32)
I dont have the book on me, but that's what I've been using to figure out how much grain I can fit in there.