Fan Air Loss

Other things equal, the amount of air you lose will go as the square of the change in fan diameter. Now if the fan speeds up because there is a smaller load on the fan motor, then that will partially compensate for the smaller diameter. If that happens or not depends on the type of motor used. You could also change fan blade pitch to compensate for the reduced diameter to maintain constant airflow.

Or remount smaller fan on smaller pulley if belt driven.

Or go to electric fan and pick your own.

As emcc says use the ratio of swept area.

ie ∏(23/2)² : ∏(21/2)² Canceling the common things on both sides gives you 23² : 21²

So the flow will be about 83% of the original amount

As long as you are not in an area of extreme temperatures, you should not notice any differences, they are designed to work in extremes of temperature, so at lesser temps, its sometimes "over done!"...