I don't know if there is an exact size, but here's what I do (note: I am a total novice too, but this works for me):
Compare your drill bits to the screw in hand - pick the bit which is slightly smaller than the main body of the screw (ie not inc. thread). Have a couple of practices on some scrap wood if you are in any doubt. If you use a bit too small, the screw can be very hard to drive in. Of course a bit too big could comprimise the holding of the screw.