I would also suggest at least a ten gallon pot. I’ll explain why in my opinion.
I got the ten gallon blichman brewpot and love it. Great investment. I now look back though and really, really wish I got the 15 gallon one.
Ten gallons is about perfect and I think the must have size for all five gallon target bottling/kegging batches. First off you will get full boils especially with all grain you want to get to the point where your getting your hot break/cold break all in the same container. (Well assuming you do your wort chilling in the brew pot) You will have excess trub to deal with, especially if you have hops debris in the pot. What this means is that I target to have at least five and a half, to six gallons of finished cooled wort in the pot, and figure on getting five and a half if possible of beer to bottle/keg. Keep in mind if you have say 6.5 or so of finished wort, you could loose up to a gallon of it due to trub/hop debris in the brew pot in excess. With just one primary fermentation I’ve gone anywhere from half a gallon to over a gallon of liquid loss due to trub/transferring to secondary and dry hop liquid loss.
I did an experiment with my last ale IPA I brewed and shot for a total target of 5.25 gallons, and tonight when I bottled I had 37 bottles, figure on 36 with the first one being a trial/air infected with the first bottling wand purge. This ended up translating in to just under five gallons in the fermenter and after transferring to the bottling bucket I had four gallons due to trub loss. This batch had a lot of extreme things for me but it is a good example of what can happen.
What that means to you? Shooting to have a finished cooled wort of 5.5 or so gallons helps you get that much closer to actually bottling 5 gallons of finished product. You need a bigger pot to help with the boils/and boil over. On my burner Ill get 1 gallon boil off per hour as a minimum, often its 1.5 to 1.75 per. This means I’m starting at usually around 7 to 7.5 gallons in a pot. There is no way with a 8 gallon pot you can really do that. (At least with ease.)
Hell I still get possible boil overs if I"m not paying attention with my 10 gallon.
My next pot will be 15 gallons and that should suffice for my needs, the reason I Want to do that is so that I Can do two 5 gallon batches of the same grain bill at the same time. This allows me to play around with using two different yeasts at the same time, or two different dry hop combinations, or secondary additions (Hmm this stout, One batch gets cocoa nibs, this other batch gets bourbon soaked oak cubes…) or any other number of combinations. I wish I could do that right now because it is a time saver as you hone recipes down. This way its all blended in the same pot to start with the same OG. (Though after that you could drain half to try two different boil kettle hop blends if you have enough vessels to do that.)
One big addition that I can’t stress enough though is to have a draining valve on the pot. I’m 32 and have a bad back from the military and can’t move several gallons of liquid around in a big pot. Having the ability to drain off the hot water with out messing with dippers/jugs for the entire thing has been great. You could get away with a auto siphon for your wort draining but my suggestion is to invest in a solid pot with a very solid drain valve.