Anyone know offhand what size fuel line I'm going to need for my 84 300D? Also, what exactly needs to be replaced on it so it's compatible with biodiesel? Just the fuel line? Anything else? Thanks!

most likley 5/16 and not 3/8. You could go to the expense of getting the "correct" (ie overly expensive) hose like Viton or just get a reasonable hose and repace it every couple years. There is not that much of it and its not hard to get to. Despite the wisdom of getting alchohol resistant hoses many run B100 with no probs.
Call around and try to get a hose rated SAE30R7. Most of the junk at auto stores will only be SAE30R6. Autozone does have hose Goodyear 30r7 hose for about 1.20 a foot. A little hunting and you can find it at an industrial supply (or e bay) for under a buck.
Even better would be SAE 30R9 hose which should def. stand up to bioD, but parts stores are going to be selling this as "fuel injector hose" for about $4+ a foot.
Just start using the BioD and then start looking for hose. You are not going to have a catastrophic fire ball where your car used to be, you may just have a clogged fuel filter or two that will tell you your line is starting to break down on the inside and its time to change out some hoses.

__________________
84 300SD 274K
38K miles on flatplate heat exchanger and various diesel/veg blends. prior to that 4K miles on unheated veggie blends with kero and DinoD.

Stock fuel line is fine for running veggie or bio. The problem with bio is not getting all the methanol out of it. If it has not been properly ater washed, to remove all the methanol, then you should not be using it anyway. Methanol is corrosive.

New lines etc. will last many years running on the bio.

__________________
1981 300D 147k
1998 VW Jetta Tdi 320k
2001 Dodge Ram 2500 141k
1979 300D 234k (sold)
1984 300D "Astor" 262k(sold)
Mercedes How-To and Repair Pictorials
I love the smell of diesel smoke in my hair