There is no ¨correct¨ venturi size, it all depends on track conditions and how much economy you want out of the motor.
The bigger the venturi (8 mm) the more power you will make, but you will get reduced run times (and probably too much power). Most guys run the 7 mm or 6.5 mm restrictors for economy and smoothness. Just tune the motor accordingly and set the EPA to match the throat size of the venturi.