If the tank is finnished, remove tank and take it outside and cap all the inlets and outlets except for vent. Fill to the top with water and close fill cap. Remove main line cap and watch water flow, it should be a constant streem, if it surges (like an old man peeing) then the vent is too small.
I ran a 5gal tank with a 7/8" dia feed tube to a 110 pump SBC blown in 7.0pro and used a 1/2" dia vent line front to back and a "T" in the middle and then forward, venting was never an issue