I have a 210 tank in my mini clubvan and that does me just fine as i would normally only go out for 4 or 5 hours per day if however i have a long day i just go home for a cuppa tea and refill my 210 tank and then back out as most of my work is local to me but if you don't want to run out or want to work 6or7 hours then i would think 350 or 400 would be best , I have just got a combo van which I'm putting a 350 tank in , my view is i would rather have the water i need and then refill if I'm getting low rather than carry 650 ltr around as the weight of all that water behind me worrys me
Just my thoughts Marc