I noticed that some physical phenomena require a system of size above some critical value to be observed. Two examples I know are:
- For a single-atom gold wire, there is a critical number of atoms to see conductance [private com., reference?]
- It was found that 60 $^4$He atoms are needed to permit molecular superfluidity to occur [http://science.sciencemag.org/content/279/5359/2083.long]
Do you know more examples?