These relations are extremely useful, and even allow us to extract exponents
and 
. But only if we are at ``large enough'' L.
If 
is very large (but not infinite), may get incorrect
``pseudo-critical'' exponents.
This is often the case at a ``weakly'' first order transition
(very small latent heat).
So it is often very difficult to distinguish between a first and
second order phase transition, or even to say for sure if there is a
phase transition at all --- is 
, or just 
?
May need extremely large system sizes to get correct results.
In general, don't know the form of the finite size scaling, so try to fit
data to a scaling function, or go to large enough systems so
that results 
constant or can do a simple linear extrapolation.