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).

Cool but how much data was it storing and what was the performance?

No idea! It’s still in its infancy as a DB. Functional, but probably not worth the switch, unless you really really don’t want to deal with scaling a DB manually (which is where I’m at.)

Right but to have a meaningful answer to your question one would need to know specifics right? As example a spanner setup that will perform on par with the largest reasonable PG setup you could have would be in 100K-150K/month range