I use a 60-quart SS pot across two burners of a natural gas stove. The pot is 18 inches in diameter and totally covers both burners of a cheap Kenmore range that I installed in my basement just for making beer. It is not without problems, however:
1. I kept setting off the carbon monoxide detector when using this setup. The final solution was to raise the height of the brew pot. I have shims about 1/4 inch thick under the burner grates. My heating/ac guy suggested this solution. Until then I would open the basement door and turn a fan on, and still have CO buildup. Now, nary a peep out of the CO detector.
2. As you can guess, with burners rated at only 6,000 and 9,000 BTU, boiling comes slowly. So when I boil, I have about twenty feet of 18-inch wide, heavy duty aluminum foil wrapped around the pot for insulation. The foil wraps over the top lip of the brewpot and extends all the way down to the top of the stove. I cut holes in the foil to accommodate the handles on the pot.
Why a 60-quart monster, you may ask? I wanted a 40-quart, but the price of the 40-quart was below the threshold for free shipping. I got the 60 cheaper after the free shipping.