I'm not saying you are necessarily doing the following, but raising the question "at what headcount" is sometimes done to suggest that this is absurd or unfair position. Why should a company with 119 employees be subject to different rules than a company with 120 employees? But this problem exists everywhere we have laws to regulate behavior. What really is the difference between 65 and 66 miles an hour? Is a hot dog really different if it has 86% organ meat vs 85%?
To answer the question, assuming we want to force 'large' companies to play by different rules specifically in the case of tolerating employee criticism, we shouldn't even use headcount as a metric. Some other measure of size should be used.