According to
https://github.com/blog/841-those-are-some-big-numbers they have 2 million repositories, getting 4500 new ones every day. But that was 2011.
On their about page they say they have +53 Mio projects hosted as of today: https://github.com/about
So, just assume that the average repository is s.th. like 20MB and do the math... sorry, couldn't find the average repository size :P
