650l tank for me too, most days I use over half of it, occasionally I'll use a whole tank, but not often, and when I do it will be a long working day.
What tank you get will be dictated by the payload of your vehicle, go for the largest tank your vehicle can safely carry.
Squeaks for instance had his tank for an escort van and transfered it to his Traffic, I'm sure that if he'd got his Traffic first he would have gone for a larger tank.
Far better to have spare capacity than not to do so.
Ian