A 5 gal pot wouldn't work for a 5 gal batch? Never done all grain before, do you not boil with a lower volume and then add water to the wort afterwards? Or is that only in extract and partial?
With all grain, you want to hit your final volume in the kettle. So to ferment 5 gallons you'll probably be looking to start boiling with 6-7 gallons (assuming a 60 minute boil, depending on your actual boil-off rate), ending up with just over 5 gallons in the kettle once the wort has been chilled, leaving the trub at the end in the kettle.
You CAN boil less, ending up with less than 5 gallons, but when you top off to hit your target volume you'll need to mix the hell out of it before taking an OG reading. Hitting your target volume during the boil means you don't need to do that at all. Plus, having more water to work with, and being able to boil it down, means you'll probably get better results from the grain.
Since going all grain, I've been doing full batch boils. Getting great results from it. I know of one other all grain brewer (BIAB method) who's pot isn't large enough to hit 5 gallons end volume, depending on the recipe boil time.
If I needed to, I could use my 60qt kettle to boil a BIG brew down to ~5 gallons. I'd rather not so I use my 32qt kettle...
Remember, your boiling to concentrate the wort, as well as process hops and such... You'll want a pot large enough to handle up to 30-32 quarts in order to do 99% of the 5 gallon batches. If you use fermcap in the boil, you can get more wort in there without having boil-over... So I could fit 30qt into my 32qt pot if I needed to... I'd rather not go that high, but I can do it. If I had just 20qt pots/kettles, I'd either be making 2.5 gallon batches, or I'd be topping off with ~1 gallon and not be able to trust the OG readings. This is supposed to be fun, so having a large kettle/pot is a very good idea.
