I've used medium size tubing on the Wasps I've bench run, but it's always good to have some small tubing on hand to sleeve the tiny fuel nipples found on some small engines.

Small tubing might have too much fluid flow resistance to feed enough fuel to a Wasp at full song.

They sell round spring clips that help if the fuel line is too large to seal to the nipple, or tank feed tube.

Soft copper wire can also be used to wrap around the fuel line, and squeeze it against the nipple/tube. Two or three turns wrapped parallel to each other should do, and then just twist the ends together to hold.

Bill