Actually, there are some issues to think about.
A small tube will have slightly (but measurable) LOWER pressure and significantly less volume.
The important issue is velocity of the fuel in the lines.
Too large a tube, and the velocity is very low (larger volume). The fuel sits in the tube and is heated by the surrounding heat sources. This can contribute to vapor lock during periods of high ambient temperature and low fuel useage (stop and go city traffic).
Too small a tube, and the velocity is very high (due to much less volume), which can cause unpredictable operation of the float in the carburetor. A side note to this issue is the Q-Jets of 1967 and 1968. The float position in the Q-Jet was changed in 1969 such that a "windowed" fuel valve seat could be used for higher volume fuel flow necessary for high performance engines. Seemed the fuel coming from the window sprayed on the float, causing a flooding condition. The repositioned float allowed for use of the window.
We would suggest 5/16 for the typical stovebolt.
Jon.
