so you want a tun large enough to accomodate ~150 lbs of grain and ~55-75 gallons of water
scaling up from my home sized rig which is 72 qt capacity and can just barely handle 30 lbs grain with ~12 gallons water you need 5 times as much, maybe a little more, around 90 gallons capacity. this should allow you to make all the way up to 1.100 with ease. and pushing it maybe 1.110.
Some pros who lack the capacity to do high gravity due to MT size double mash. mash half the grain, drain, bring the wort back up to strike temp, refill the tun with the other half of the grain and mash using the previously mashed wort as hot liquor.