3/8" to 3/8" is the correct size.
Is it leaking because of failing O-Rings, or is the connector cracked or damaged?
I ask because replacing the 2 O-Rings inside of the connector may be easier then replacing the connector itself, that and with this is with that connector I am not sure you will be able to get the connector to connect back on the fuel filter, because once you cut the old one off, your line is going to be about an inch or so shorter than it was.
I had to do some repair on my fuel line not too long ago, and I had to go with another solution, of course I had a pinhole in the line not far from the fuel filter area.
This is what I went with, I actually bought two of them so I'd have a new connector on each end, as sometimes just removing these can cause leaks, not in all cases, but it is possible.
You can cut the line a bit further back from the filter, and cut the replacement line so it is still the right length.
Note, it was not easy getting the old line attached to the brass connector, you need to find some way to get the line warm enough so it expands, and slips down on the fitting, of course you cannot use fire, so the only thing you can safely try is some hot water.
Is the line that is leaking on you the one that is about 2 feet or so in length?
If so you could take it off, take it inside, let it air out a minute or two, and use a hair dryer, or heat gun, I personally quickly passed mine over the flame of a propane torch, but you have to be quick, and careful, or you can ruin the line/connector.
Here is the thread I made when I had my issue, sorry there are no pics in it, but there should be a helpful tip or two.
Repairing leaking nylon fuel line.