I use a 15 gallon aluminum stock pot for 5 gallon batches, well six gallon batches. The diameter of the pot is about 16" if I can remember correctly. I tend to boil off about 2 gallons on a hour long boil. I enjoy this cause I think it helps my efficiency. So my batches i gather 8 gallons of wort boil down to 6, siphon out 5.5 to the fermenter. So yes, the largest surface area will increase evaporation but is that really a problem? Also, boil off is a constant, so if you wanted to do say an 11 gallon batch on my system you would boil 13 gallons to start, leaving 2 gallons headspace, which could be pretty tight, but definetly doable.
Side story - My third all grain brew I thought I would start to heat my roughly 2 gallons of first runnings outside in the 15 gallon pot while i finished sparging inside. Gather my second runnings in a bucket and went outside to add it to the boil kettle and I was JUST IN TIME! Those two gallons almost boiled over my 15 gallon pot. I was shocked. So keep an eye on that pot before you have hot break.