There are a number of large repositories in existence.
Mozilla
https://hg.mozilla.org/mozilla-central/ is the mainline of Firefox development. It has over 200,000 changesets and 100,000 files.
https://hg.mozilla.org/releases/mozilla-beta tracks the releases for Firefox Beta. It has 600+ topological heads and 1500+ tags.
https://hg.mozilla.org/users/gszorc_mozilla.com/gecko-aggregate/ is a pre-production repository with lots of interleaved commits between heads. It is an aggregate of most of the Firefox-related repositories. It is an excellent repository for performance benchmarking Mercurial. Note that when this page was written, hg.mozilla.org was not using bundle2 and gecko-aggregate was using generaldelta, so clone times will likely be very long.