Tug,
Measure the outlet fitting on your gas tank to find the outer diameter (OD) of that fitting, and then use that for the inner diameter (ID) of your fuel line. Also check the OD of any fuel filters and the inlet fitting of the outboard(s). They should all be the same, but it would be wise to check.
Figure out how much hose length you need and then buy an additional 50%. Unless you have a hose stretcher. Sorry, I loaned mine out and the guy never returned it.
Boris