widely assumed 5nm is the limit. I know i've seen others discuss some ideas around how close they can get, but i'm struggling to find the thread.. In any case, this may help: https://en.wikipedia.org/wiki/5_nanometer

Found it.. http://semiengineering.com/will-7nm-and-5nm-really-happen/ and the HN discussion from several years ago: https://news.ycombinator.com/item?id=7920108