According to an Oct 2011 answer by Zach Holman on Quora[1], they only release publicly the number of users. According to the homepage, that number is currently at 1,455,656 (hosting 2,444,052 repos). You could extrapolate from their with an average repo size, but I haven't seen any good information on what that size would be.
[1] http://www.quora.com/What-is-the-size-of-GitHubs-userbase