Its always nice to have have spare capacity within the tank...having a 500litre tanks does not mean you need to fill it...but I started with a 250 litre and wished I had bought the larger tank straight away.
Cheers
Dave.
Reading another post (glad I did) what concerns me about getting a larger tank in the car (a peugeot 406 T reg) is: Would the weight of the water of a 250 or 500 litre tank effect the steering/brakes/suspension and would it be unsafe not being bolted down if it was in the boot?
1/2 a tonne of water sloshing around in the boot?
Ask your insurance company, see what they reckon.
Steering, you wont have any.
Brakes, that'd be interesting.
Suspension, equally interesting, have you got a camera and are you fully comp?