HN user
There is already some work towards this with http://www.npgsql.org/
I haven't gone back to try the full entity framework recently, but if you find any bugs, please let us know. If you want to roll your own, we're here to help too.
It's on our roadmap and is actively being worked on right now. Should be available for version 1.2. See: https://github.com/cockroachdb/cockroach/blob/master/docs/RF...
I can't talk about the largest production database, but our largest internal cluster we've tested on so far is 128 machines and hit each one with our own continuous load generators and a chaos monkey (who turns on and off machines at random).