Is a small Cross still a Cross?
The Muslim Americans have their rights and sensitivities, too. As do Jewish, athiests, etc.
Then there's the Wicca, and God knows what-all.
Don't know that a police car with a Cross on it is a great idea. "To serve and protect" kinda takes on a different meaning when there's a Cross there.
I would think it's not the size of the Cross which could be an issue, it's the quality of the Cross, what even a little one represents.
Arf