I use the coleman xtreme 52 quart (see link). Right now I am only doing 5 gallon batches but plan to move to 10 in the future which it will still handle. I did not lose more than 2 degrees all winter even with the amount of head space left from doing a smaller mash.
http://www.kmart.com/coleman-52-qua...W014013110001P?prdNo=1&blockNo=1&blockType=G1
On a mash tun, I have the same philosophy as I do on a kettle.
Use the largest damn thing you can find.
I got a Coleman Extreme 72qt. There are a number of reasons I like that size and shape, but one of my favorites is that I don't normally have to wonder if I csn fit any amount of grain and water in there.
One less thing for me to worry about.