I use this tool all the time to see what I can and cannot do.
http://www.rackers.org/calcs.shtmlMy Imperial stout is about 23.5 lbs of grain. To do a double batch at 1.25 qt/lb (slightly thick in my opinion) I would need at least a 75qt (18.75gal) cooler, I have a 70 qt now so could maybe go extra thick and push it, but it would be very full.
Nothing wrong with going as big as you can and adding extract to the kettle.
In my opinion, it is best to get the vessel that will work for your needs 90% of the time and adjust for the other 10%. For this reason I have two cooler tuns, one about 40qt and the other 70qt.
I do think it is best practice to brew a large amount of big beers. 5 gallons tends to go too fast and I never was able to drink a proper 3-7 year old bottle. Right now I do have one bottle of 6 year old imperial stout that is stashed in a box at the bottom of a pile. I have sense re-brewed it so I am not tempted to drink it.