Found the answer here, referencing a posting from 1997:

http://vw.upnorth.net/Replacing-Fuel-Lines136.htm

10mm :)

On 7/17/05, Joel VanderKwaak <joelvanderkwaak@gmail.com> wrote:
> Howdy,
> 
> Anyone know the specs (od, id, etc) for the short piece of fuel line
> going from the fuel tank to the fuel pump (1991 gasoline, if that
> makes a difference)? My van is on loan for the weekend, so I can do
> any measuring.
> 
> Thanks,
> 
> Joel
>