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/experimental/firefox-unified contains several heads of Firefox in chronological order. It is using generaldelta + aggressivemergedeltas.
https://hg.mozilla.org/users/gszorc_mozilla.com/mozilla-central-aggressivemergedeltas is a generaldelta version of mozilla-central with aggressivemergedeltas enabled. The repository has some very long delta chains and is appropriating for benchmarking manifest read performance for very long delta chains. Use hg clone --uncompressed to preserve the very long delta chains in your local clone.