If you want to man handle it into the back of a truck then 10 gallon MAY be max. If you want to pull it with a vehicle slowly to a dump station a huge one may be fine. I have two 15 gallon that I can roll up my drop gate utility trailer and pull right up to any dump station and also carry a 60 gallon water bladder on return trip.
My older rig doesn't have a belly liner so both of them are bungeed up underneath the rear cavity out of site. I have seen where people will hang them on a rear ladder and write something like "Meals on wheels" on the tank but I'm not into that.