I can't advise as, 1) Its an AZU van, 2) I installed copper tube where I thought it should go and probably not Citroen.
However, for me it was 1.5 Metres of 6mm dia however others say 5.6mm is the correct dia if you can find it.

The only spec. worth having now is DIN 73379-3E, available at ECAS 2CV Parts, of course,

and ebay here http://www.ebay.co.uk/itm/100-BIO-Fuel-Hose-Petrol-Pipe-Ethanol-Butanol-Methanol-Diesel-Cohline-R9-R6-R10-/251182149018?var=&hash=item3a7b9f6d9a:m:maMS093uMOcKpDlojI3bD6A

and from Germany here https://www.landefeld.de/artikel/en/biodiesel-fuel-hose-din-73379-3e-55x115mm/KRAFT%20612%20OKO

I was under the car one day looking at the rear silencer when I noticed a flimsy green hose with a sleeve over top of it underneath the car under the driver's seat (LHD). Just looking at it, it looks like some sort of vaccum hose. Very soft and small diameter.

Someone told me this is the fuel line?!? How much of this green poly hose is there on the car? What size is it? This stuff looks like a leak about ready to happen.

I'd imagine there would a metal pipe from the tank to the engine. Is it a continuous hose?

In the engine compartment the fuel hoses looks like ordinary fuel line hose.

For what it's worth, if you guys can find Gates LOL hose in the UK, you can use this stuff on just about anything. I've used it on E10 petrol, engine coolant, engine oil (turbo drain hoses) and it has held up. I've never used it on paraffin or diesel but I'd say it would work. I think it's made of neoprene.

None.
This is what it should have:
http://www.ecas2cvparts.co.uk/petrol-pipe-clear-nylon-black-45mmx6mmx3mtr-goes-along-chassis-rail-from-front-rear-p-71.html

That's probably it. For some reason I thought mine was green. The thing is mine feels soft like a wet noodle.

That description suggests it might be on its way to failure.
The correct pipe (hard nylon, 6mm OD) seems to be just about everlasting, since the one on my 1971 Dyane is still fine despite years of exposure to unleaded petrol, which in the UK is laced with about 5% ethanol.
It always beats me when folk think that they know better than Citroen...

The green soft one sounds more like aquarium tube to me. Someone did a easy repair? Even my 57 models had this nylon tubing, still going strong.

I'll check again. I was under the side of it yesterday and found the hard poly tubing running the length of the car (covered in paint of course). I'll try to take a photo of this hose I was talking about.