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.