i believe it's 1/4" inside diameter, outside diameter close to [if not actually] 5/16".
it actually has to be a vacuum type hose, because when the radiator cools down, the fluid is sucked from the overflow tank back to the radiator. that is why these cars require a cap with two seals. one for the top of the neck, the other to seal against the necks bottom at the top of the tank.
parts store may have this on a bulk roll. if they don't, i may have a piece in my stash that will work for you. gratis.