1. Saeed is writing a really nice series of articles starting here: https://www.visualstudio.com/learn/git-at-scale/ In the first one, he lays out how we think about small/medium/large repos. Summary: size at tip, size of history, file count at tip, number of refs, and number of developers.
2. Fairly confident, at least as far as usable repos go. Given how unusable the Windows repo is without GVFS and the other things we've built, it seems pretty unlikely anyone's out there using a bigger one. If you know of something bigger, we'd love to hear about it and learn how they solved the same problems!
3. Windows is a 30 year old codebase. There's a lot of stuff in there supporting a lot of scenarios.
As far as I can recall, this is not possible using Windows source control, as its history only goes back to the lifecycle of Windows XP (when the source control tool prior to GVFS was adopted).
Microsoft does have an internal Source Code Archive, which does the moral equivalent of storing source code and binary artifacts for released software in a underground bunker. I used to have a bit of fun searching the NT 3.5 sources as taken from the Source Code Archive...
2. Fairly confident, at least as far as usable repos go. Given how unusable the Windows repo is without GVFS and the other things we've built, it seems pretty unlikely anyone's out there using a bigger one. If you know of something bigger, we'd love to hear about it and learn how they solved the same problems!
3. Windows is a 30 year old codebase. There's a lot of stuff in there supporting a lot of scenarios.