Some companies will never be big enough. Some companies are too small. Very few companies are just the right size.
The only thing that matters is not your current size, but your ability to bring together a team to deliver a specific product or service to a specific customer at a specific point in time in the future.
The past does not really matter though it can be perceived to be proof of the future by many buyers. But the past is not today, nor is it the future.
The work can be done in-house, contracted out or through a consortium. The quality of the team for this project is paramount.
So long as the customer is happy at the end of the day... that is big enough.
Update - 29 April 2009: Seth posted a good related article. Maybe big and great are not the same.