I thought we had done this in another thread but......
No 1000 ltr isnt to big physically, i have one in my vito. You need to check the load weight of the van your looking at to see if it is suitable ( i dont know if mine is but i v.rarely carry it more than 1/4 full).
You only need a big tank if you have no water supply, but you wont do much more than a couple of terrace back yards with one tank approx (1hr).
In short.
If you want to carry a water tank work out the weight of 1000ltr, find a van thats big enough and will carry it safely and buy one.
I am not sure what else their is to say..
one thing,
Dont forget the size of tank is irrelevant unless you have no water supply. so i would get either a smallish one to use as a holding tank so you can work your gear from the van Or a large one if your going to work were there is no water supply. But i dont see the point of a medium sized tank cos its to big if you have water supply and not big enough if you dont.