Calculating Lattice Energy: LiF vs. CsF

[outofmines]

I'm having some problems with the calculations associated with this problem:

Which compound in each of the following pairs of ionic substances has the most exothermic lattice energy?

(a) LiF
CsF

I know I need to use Coulomb's law, but I don't know how to calculate r (the distance between the ion centers)

Any help is greatly appreciated. Thanks

 

[outofmines]

nevermind, I have found out that bigger ions have a bigger r value and thus via the inverse proportionality the higher lattice energy corresponds to the lower r value