How do ionic charge and radius affect lattice enthalpy?

As the charge of an ion increases, so does its charge density. The ion can therefore form stronger electrostatic forces of attraction, resulting in stronger bonds. As these bonds form to produce a lattice structure, more energy is released compared to from ions with a lower charge. This results in a more exothermic (more negative) lattice enthalpy.
Conversely, as ionic radius increases, charge density decreases, resulting in weaker bonds during the formation of a lattice so the overall enthalpy change is less exothermic.