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I am trying to learn about the total number and breakdown of small, medium and large companies using Google Adwords for search marketing. If you find out, please site the reference. Thanks!

I would try going to the google adwords page, and asking them. I'm sure there is a contact us button there for people who have questions.
It's gonna be hard to find a total breakdown without them, so I think that's gonna be your best bet.
Hope this helps!