Hmm... well I am not sure what you consider to be "big"

(what DO you consider big... out of curiosity... I am no vB veteran or anything), but here is my "biggest" one at this time...
http://www.womens-health.com/boards
Threads: 2,139, Posts: 6,676, Members: 3,424, Active Members: 594 and I think the most ever online at once was something like 125.
We are averaging around 4,000 unique visitors per day and somewhere between 20,000Mb and 44,000Mb of bandwidth per month. So somewhere around 1,000 Mb per day give or take and we aren't having any problems with it.