I'm recommending that you be open and honest with your revenues as they fall into the context of a potential sale. Once you've been able to pull all of the information together (year over year revenues, adoption rates as compared to the total market, etc.) together and undergo an audit for the true value of the business, you'll be in a better position to negotiate a sale to one or more parties.

If your suitor doesn't come right out with a great offer and their own NDA accompanying it, which should go hand in hand, I wouldn't recommend entering any type of secrecy agreement with them and open up a potential sale to other suitors, if they do exist. If this big company you speak of will not prove to you that they're serious at the outset, it's probably just a cursory fishing expedition to begin to assess the market and you'd just be a starting point, so yes, I'd keep my pants on until you know they're serious. After all, they're approaching you. Let them do the talking for awhile.

But the best piece of advice which is echoing in the thread is to lawyer-up asap, and don't go buying that new piece of shiny consumerism just yet.