If you use 6mm O.D (4mm I.D) pvc tubing up to the pump and make sure all the tank fittings, clunks and tubing are also 4mm I.D you won't go far wrong. The only time I would be happy to have a bore less than 4mm is either when I use a small engine or I have two parallel tanks of the same size feeding a hopper which has a 4mm I.D feed to the UAT.

Jason