I think of it as more of an attitude. At 500+ lawyers, the biglaw attitude is definitely there. However, and perhaps unfortunately, that same attitude can thrive and flourish at a firm with 150 lawyers. (Hell, the biglaw expectations can even exist in a 30 person firm, though those firms are obviously called boutiques.)
Atomic40's definition is the most practical, I think. Does the firm run around the clock? Are there expectations that people will step out of dinner to take a call, and leave dinner if necessary to return to the office....even when it's NOT two months before trial or before a deal closing? Are support staff around to assist with weekend projects? Are there little amenities to make the long days livable, like dinner or taxi service?
A quick litmus test is to determine people's relationships with their Blackberries. If it has to stay on, 24/7, there's a biglaw sensibility lurking there. If it can be turned off or left uncharged without huge guilt, it's probably not biglaw.