What we talk about here is Google's search database. Giving a near exact figure on how big Google's search database is, is very difficult to do. It's growing each millisecond and even if we were allowed to monitor it (everything related to the Google Search database is a corporate secret and only a few people know the real details), we would need a real-time counter.
Based on the information we -do- know there are some estimates however suggesting that the Google Search database (2013) is around 450 to 550 terabytes. And that's "just" the search part only. Imagine adding to that Drive, Picasa, G-mail, Blogger, Youtube and all the other Google services. I think we would be looking at more then 80 petabytes of Google at the very least!