what size fuel line do i need to get to replace an old line on my 20hp gdlt? where can i get like 10-12ft of it?

1/4" and lowes hardware sells it by the foot.

I got mine from a place that sells gaskets and rubber hoses don't really remember the name, It is 1/4" high-temp/non-corrosive high pressure fuel line that the NHRA guys use on there high powered dragsters, shoot the boat will rot away around this stuff before it does, really expensive. $3.25 a foot and i got 18' so you do the math. Oh and its a light blue color.

I just got mine at Checker/Oreillys auto parts. but I'm sure most auto parts places carry it..

Where did y'all get the fitting that connects the line to the tank at?

just ask for some hose clamps when you get the hose.