"BIGLAW" I believe, is any lawfirm with nationwide offices/recognition - This entails offices in the big cities - Chicago, Boston, New York, Washington, DC, Los Angeles, Atlanta, etc.
Some more regional firms might have 300 attorneys, but they have more of a regional reputation and client-base. These firms can still pay next-to-market salaries, though they're usually a bit below it, but I don't think they're considered "BIGLAW".
That being said, some firms, like DLA PIPER, have 3,700 lawyers globally. Now, if that's not big law, I don't know what is.