If you want to have better aero the audi ultra models at least have these kind of flaps. Again the heigth is limited to my estimation by usability of the car to certain level. Air when hitting a vertical plate cannot decide itself which way the air should go. It will go to way which is easiest. Thats why bigger flat plates are not always better as you can find results and test to prove that claim.
If you drive the air pass the tire in controlled manner you will see better results. That is the purpose of that front facing rubber piece. It was about the same width as the tire. If you have more panels in your suspensions arm the other flap does not need to be so wide. Only to cover the tire surface area.
New range rover evoque has flaps that are just the size of the wheel. I would estimate they would come more towards outside if aestheticts would not ban that. On the inside they are exatly on the edge of the tire. That I got from online aerodynamic seminar of range rover development.
