  We study the generalized synchronization and its stability using master
stability function (MSF), in a network of coupled nearly identical dynamical
systems. We extend the MSF approach for the case of degenerate eigenvalues of
the coupling matrix. Using the MSF we study the size-instability in star and
ring networks for coupled nearly identical dynamical systems. In the star
network of coupled R\"ossler systems we show that the critical size beyond
which synchronization is unstable, can be increased by having a larger
frequency for the central node of the star. For the ring network we show that
the critical size is not significantly affected by parameter variations. The
results are verified by explicit numerical calculations.
