wilserbrewer said:
While the 70qt should / may work fine...on a five gallon batch w/ 10 lbs of grain mashed at 1.25 qt/lb is just shy of 12 quarts...12/70 is around 17%, or less than 1/5 of the tun will be filled w/ mash, seems a little excessive to me?
Not sure how you're doing your math, but less than 12 quarts is wrong. It'd be 12.5 quarts just in water, plus the additional space taken up by the grain, which for obvious reasons is not simply additive, but definitely increases the volume. Promash puts the additional grain displacement at about 1/3rd quart per pound, so 10lbs would be about 16 quarts total.
Not a huge difference though. I only have a 50q cooler (Coleman Xtreme Ultimate), but I don't plan on doing 10gal batches any time soon. The OP does though, and I think it's much more important to have enough space than it is to make sure you don't have too much.
Two main concerns with having too large a cooler:
First is that the water might not be high enough above the grain bed and will cause channeling. This is ONLY an issue with fly sparging though, but I don't think the OP said whether they batch sparge or not.
Second issue is having too much empty space and seeing temperature drops of a few degrees as a result. It's usually not that dramatic, but still... it's not unreasonable to want very stable temps. Luckily, there are a few ways of dealing with this, if you decide it's too big an issue. Some people put blocks of foam underneath their lid to minimize the empty space, while others literally just place (safe) things in the coolers to take up additional space. Bottom line is that there are always things you can do to, in effect, make your cooler "smaller", but you can never make it bigger.