I have seen a Duece coupe with windows about four inches tall on the side on the street, it doesn't mean it is right. My truck has 12 inch high windows and they are a pain sometimes.
As far as legal? That depends from state to state. Honestly, it is one of the oldest "old husband tales" out there. In California there is NO rule, NONE in the vehicle code book. I have one right here in front of me, there is NO such rule. I have heard throughout the years all kinds of "old husbands tales" but there is no such rule. There is ONE thing refering to "windshields" that could be construed as a "height" requirment. It states "Must have adequate visibility". This of course leaves it up to the officers discretion. Which means he could give me a ticket for my four inch chopped 48 Chevy if he wanted to.
Let me put it this way, I have never, not once heard of a guy getting a ticket for chopped top.