  We study the dependence of the retrieval parameters, such as the electric
permittivity, the magnetic permeability and the index of refraction, $n$, on
the size of the unit cell of a periodic metamaterial. The convergence of the
retrieved parameters on the number of the unit cells is also examined. We have
concentrated our studies on the so-called fishnet structure, which is the most
promising design to obtain negative $n$ at optical wavelengths. We find that as
the size of the unit cell decreases, the magnitude of the retrieved effective
parameters increases. The convergence of the effective parameters of the
fishnet as the number of the unit cells increases is demonstrated but found to
be slower than for regular split ring resonators and wires structures. This is
due to a much stronger coupling between the different unit cells in the fishnet
structure.
