How large is github. Is there a legit way to download it all?

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

I don't have an answer to your question off the top of my head but I am curious as to why you would want to download all of the open source code on there?

Well, I thought I naturally could download github like I can with Wikipedia. The basic curiosity I had is whether someone (not necessarily myself) could do some pattern magic and autocomplete from all the available corpus. CRUDs would be strokes away, loading off the internet animations, mathematical formulas..

I think it's just plain interesting what you could do with all that code.

Not so large.