When I was at FB, there’s no PR as defined at GitHub. Each independent commit was a separate review. So in that sense, all of the 10s of thousands of engineers at Facebook daily review “PRs” at the commit level.
Wip commits we’re often and frequent for CI purposes but as with PRs you don’t want to waste too much review time on WIP unless your asking for some preliminary feedback on a complex change.
Ideal commit length was unchanged to any other commit length you might be expected to do on any other team with good commit hygiene.