A good-sized air inlet and air outlet in your cowl doesn't guarantee good cooling. You have to make absolutely sure that the air entering the cowl actually passes THROUGH the fins of the cylinder and head of the engine. If you look into the air inlet and can see the air outlet, then the air's following that path and not cooling the engine. With larger models that have large cowls, you have to make baffles to make sure the air entering the cowl acutally passes throuth the engine's fins. The Great Planes Lancair ARF kit comes with baffling you fit to the cowl and engine to make sure the engine gets cooling.

When properly-baffled, you'd be surprised at how small an opening is needed. On a full-size RV-9A homebuilt, the cowl air inlets are about 3" x7" each (or slightly larger) to cool one-half of a 160-horse engine (two cylinders). It's making sure the air's put to work that's important.