Just out of personal curiosity, how much disk space on the server does Stack Overflow take up? I'm guessing it's a lot, but I can't know for sure.
It was 12gigs compressed a few months ago, probably up to about 15gigs compressed (50 gigs uncompressed)
It is not a particularly huge database, most of the storage we have is taken up by history and indexes. Fitting most of our data in RAM is one of the reasons our performance is so awesome (that and Jeff's amazing wheel of blame).
When you look at perfmon: SQL server page life expectancy is massive, we barely go to disk.
This is an ever increasing number, so no answer would be correct for long. However, you can use the Data Explorer to estimate the number. The following was accurate in the second half of August 2014.
The Data Explorer does not include the 15% of posts that are deleted (which stays in the real database) or all of the post history, so the actual database size is much higher. Thanks to to Weapon of Choice for this information.