For my Zetec installation, do I need an 8 mm return fuel line, or do I need 6 mm? Or doesn't it matter?

I don't think it really matters a lot, but most cars are piped in 8mm (or 5/16" bore flexible pipe and 8mm (or 5/16" OD hard lines, so one will fit over the other.

These are the sizes I use for everything except the feed from the tank to the pump inlet (where I use 12mm or 1/2"

Thanks, reading the Luego build manual (which is pretty good!) gave me the same numbers.