Lattice energy of an ionic compound depends upon:

According to Coulomb's Law and the Born-Landé equation, Lattice Energy ($U$) is given by:
$$U\propto \frac{z^{+}{z}^{-}}{r_0}$$
Where:$z^{+},z^{-}$ are the charges on the ions.$r_0$ is the equilibrium distance between ions (related to the size of the ions).Thus, lattice energy depends on both the charge and the size of the ions.