  Cluster molecular field approximations represent a substantial progress over
the simple Weiss theory where only one spin is considered in the molecular
field resulting from all the other spins. In this work we discuss a systematic
way of improving the molecular field approximation by inserting spin clusters
of variable sizes into a homogeneously magnetised background. The density of
states of these spin clusters is then computed exactly. We show that the true
non-classical critical exponents can be extracted from spin clusters treated in
such a manner. For this purpose a molecular field finite size scaling theory is
discussed and effective critical exponents are analysed. Reliable values of
critical quantities of various Ising and Potts models are extracted from very
small system sizes.
