I've seen a lot of the cube style coolers that allow for a large grain bed, so that could be an option as well. Just be sure that you can get your mash paddle into the corners.
In terms of size, check out the following. You can also search for it.
Originally Posted by Bobby_M
There seems to be a huge uprising in the number of new all grain brewers and the persistent question is "how big of a tun do I need?" It depends on what your batch size, efficiency, and desired OG is. Here's the easy way for you to decide based on your needs.
Keep in mind that even if you think you might want to do both 5 and 10 gallon batches, odds are your really high gravity beers will be 5 gallon batches. Who needs 10 gallons of barleywine?
Just for example, a 10 gallon mash tun can do 5 gallons of 1.118 barleywine (~11% ABV) and 10 gallon batches of 1.060 IPA (~6% ABV). I personally think something in the 50-60qt range is most flexible in that regard if you don't mind a square/rectangular cooler.
If you want to figure for larger or smaller batch sizes, take the number in the "Max Gravity Units" column and divide that by your desired batch size.