Air intake size
If you are referring to induction air, I just finished researching that. For the O/IO 360 engine, at 2500 RPM and 180 MPH it seems that 2.2 inches in diameter is the minimum number... rounded up to 3 inches would give about I inch ram increase without inducing much drag. Larger is not necessarily better here, as the drag would increase with a larger duct.
If you are talking about engine cooling air I would refer you to the DanH "shrinking exit" for exhaustive discussion.
Others, more knowledge than I, might chime in here.