When I ran my Warrior 165 with a 60hp 4 stroke Mariner, I used to carry 50l, about 11 gallons. I used just about 0.75l per mile so this gave me about 60 mile range. When wrecking mid channel, I used to take an extra 25l.
Outboard tanks will have one of maybe 3 or 4 different connections on them. Provided all your tanks have the same connection you can simply unplug and switch tanks, this saves decanting fuel at sea.
I'd suggest you need minimum 5 gallon (25l) tank, ideally 2 of these.
Hope that is helpful.
Adam