The brewhouse at McPhillips Brewing is optimized for five gallon batches, but it’s not unheard of to brew a ten gallon batch now and again. Case in point, this ten gallon batch of IPA that clocked in at an original gravity of 1.060. Between the twenty-five odd pounds of grain, plus the water required for mashing, my ten gallon cooler mash tun was full to the top. I actually mashed in a little thick and made up the difference on the sparge. Last time I had the mash tun this full was for a five gallon batch of Russian Imperial Stout.
I have a Sanke keg that I’m planning on building a new mash tun around, which should make high gravity five gallon batches and regular ten gallon batches a little more manageable. For now though, I’ve found this page of brew related calculators pretty helpful.