Criminal Sentence 673: How big of chunks?

A quote from someone in yesterday's newspaper, in an article about workers trying to stabilize a giant rock that is poised to fall:

"We don't know how big of chunks are going to fall."

That's a big chunk of strangeness in my opinion. I understand what the person meant, but that does sound very odd, doesn't it?

He probably should have said something like this:

"We don't know the size of the chunks that are going to fall."

I realize that regular joes don't always speak grammatically. Perhaps I can let it slide in conversation, but nobody should write a sentence like that.

The construction "how [adjective] of..." works with singular nouns, as in "How crazy of a person are you?" But as we can see here, it doesn't work with plural nouns. I don't really know why. Any theories?