Here is a link to a chart showing the various AN (Army/Navy Specs) fittings:
http://www.gre6.com/ansizeThing with AN fittings you can use what ever size you want and select the adapter fittings based on the pipe thread size at the final connections. For fuel lines I would go with AN6 that uses 3/8" ID line. You could use AN4 however on the small size IMHO. I do not thing the small cost savings is worth possibly not having the flow rate needed at WOT. I buy most of my AN stuff on Ebay. There are tons of this stuff there as NASCAR rules only allow them to run lines for a specified period of time. You can get stuff 50 cents on the dollar or less.