It's an I'll posed problem and Held has a good post on extreme sensitivity to computational domain size. (planet3.org)
The thin films displayed a minimum domain size of L0 = 11 nm. (european-mrs.com)
A certain number of residues is necessary to perform a particular biochemical function, and around 40 - 50 residues appears to be the lower limit for a functional domain size. (sciencedaily.com)