That size is big enough to do an AG batch, but just barely. I typically start with 7.5 gallons in the brew kettle to get a 5 gallon batch, and that pot would only allow for small amount of bubbling during the hot break. I have a 48 quart kettle and do not usually have boil over problems because of the extra space. Trust me, when you do AG, you will have a lot more hot break than with extract. I used to boil in a 30 quart pot, and I never did a full boil in that one, as I would boil over.
I see the 56 quart size of the same item is $20 more. Personally I would spend the a little extra money on that.