They are called restrictors, they limit the air intake. A larger opening means more air and that means more fuel is needed, ofcourse that will give more power.

A smaller one does not give that less power but it can be noticeable and can save you a fuelstop in a 5 minute qualification. In this situation you have to decide whats faster.
Just start with the largest one.

On the edge there is a number, the black is 9mm and the red must be 7 or 8mm