What ever fits, seriously. If small is too hard to get on the fittings, use medium. If that's too loose, safety wire it. You only have two choices, small and medium, and you don't even wanna think about large.
   Type at you later,
  Dan McEntee
   

I always used surgical tubing. That nice brown surgical rubber. Get the small tubing and it is a perfect fit. Downside is that the fuel rots the tubing and you will replace it on a regular basis. It was the stuff that was always used in 1/2 a speed when I was a kid in the year ....BC. Check surgical supply sites or nearby stores. Also the really small aquarium plastic tubing works. Same problem though as it must be replaced on a regular basis. The fuel makes it rock hard and if you have to take it off the fuel nipple you might as well replace the tubing