For a 5-gallon AG batch, most people start with a 5.5 gal recipe to make sure the full 5 gal makes it to the fermenter after racking losses, samples, etc.
Given a 5.5 gal batch size, you'll start boiling with somewhere between 6.5 and 8+ gallons of wort depending on the beer style, your system efficiency, desired boil length, evap rate, etc.
You can squeeze by with a 7.5 gal turkey fryer (many people here do).
You'll have fewere headaches and more flexibility if you go to 9-10 gallons.
If you think you'll ever want to do 10-gallon batches, might as well go immediately to a 15+ gallon pot, or a keggle. Otherwise you'll be having this discussion again
Use the old pot as a hot liquor tank (HLT)