Too small? That definetely can't be the reason unless the guy knew something was wrong with his car wash and smallER cars were having trouble in it.
The Boxster is small, but it's not freaking CRAZY small or something.
Now, I think the guy maybe didn't want the liability of a Porsche in his car wash.
OR, maybe he was messing with you and referring to the size of your penis because of the car you drive. After all, he did say "too small!" not... "your car is too small!"