If I recall correctly, 1 lb of grain saturated with 1 Qt of water has a volume of 1.4 Qts. This is from How to Brew. I always forget the amount and have to hunt around in there before I find it. Any additional water you add just adds volume. So, depending on your desired mash thickness, you should be able to calculate the most grain you can put in.
Here's a quick calculation I made on the maximum lbs of grain you can fit in various mash tun sizes. Please check these for accuracy as I'm at work right now and don't have a copy of How to Brew handy. It doesn't account for any volume you may have beneath the false bottom.
Qts 1:1 1.25:1 1.5:1 2:1
40 28.6 24.2 21.1 16.7
60 42.9 36.4 31.6 25.0
80 57.1 48.5 42.1 33.3
100 71.4 60.6 52.6 41.7
I have a 20 gallon kettle and Mylo and I made 14.5 gallons of 1.100 Russian Imperial Stout wort in it with 55 lbs of grain at a 1:1 water ratio. That was one frickin' full mash tun.