[Aussie Mike]

If you aren't running a full return system you probably don't need such a big fuel line. With a carburetor car you have the volume of fuel in the float bowls to take up the slack whilst the pump catches up.

For an injected car my theory is (and this might be total rubbish) that you need to keep the feed line large enough that it can flow any instantanous demand. The pump is providing that head of fuel back at the tank and it may be able to empty a swimming pool in 2 hours. That may be no use if the line won't flow enough that you get a pressure drop at the injectors when you put your foot down. The pressure drop might only happen for an instant but that will translate to a potentially lean mixture.

I reckon it's much the same effort to put in a 5/16", 3/8 or a 1/2" line and not that much difference in cost. Bigger is better.

Cheers