Using the Surface Forces Apparatus on solutions of polymeric ions, the effect of specific ion-ion correlations is evaluated on the characteristic decay length, λ, of screened electrostatic interactions between charged surfaces. Electrolyte solutions composed of point charges surrounded by repulsive polymeric shells were used to elucidate the role of ions size and size asymmetry between co- and counterions on the screening of electrostatic forces. In electrolytes composed of large polymeric cations and small point-charge anions, because of the steric and excluded volume effects, the screening length follows the simple scaling relation λ ∼ d, where d is the characteristic size of the large cation. It is also reported that both co- and counterion sizes affect the thickness of the electrical double layer and influence the screened electrostatic interactions. In solutions of polymeric cation/anion pairs, the screening length is shown to depend on an asymmetry factor. These results provide insights into correlation effects in electrolytes, which were so far unreachable experimentally and help elucidate such effects in electronics, energy storage devices, and biomedical systems.