F = k*q1*q2/(r^2)
The magnitude of ionic charge is the charge on the cation/anion.
If the magnitude of ionic charge increases, then q must increase because q is the charge. Therefore, the force between two charges increases, and the potential energy of the system decreases (bonds get stronger due to more force). This makes the lattice energies more exothermic.
The magnitude of ionic charge doesn't have much to do with the dist. between atoms. If the dist. between ions increases, as component ions get bigger and therefore farther apart, then the quantity "r" in coloumb's law increases, and the force decreases, making lattice energy less exothermic.
Therefore, the highest lattice energies will be between highly charged, small ions (aka BeO), and the lowest lattice energies (least endothermic) will be between large, less charged ions (aka CsI).