Based on a recent formulation of the V-parameter of a photonic crystal fiber we provide numerically based empirical expressions for this quantity only dependent on the two structural parameters - the air hole diameter and the hole-to-hole center spacing. Based on the unique relation between the V-parameter and the equivalent mode field radius we identify how the parameter space for these fibers is restricted in order for the fibers to remain single mode while still having a guided mode confined to the core region.