I don't know about fluxing power, but if it's ion size you're after, that's a simple function of the number of protons & neutrons in the nucleus, as well as the number of electrons in the various orbitals.
A periodic table of elements is all that's necessary
http://www.webelements.com/
In essence, down is larger in any column, so it's Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Cesium (Cs), Francium (Fr). Also, sideways is also larger for a given row. (i.e. Magnesium (Mg) is bigger than Sodium (Na).
Does this gibe with fluxing power (i.e. Sodium larger than lithium, etc)?
Cheers,
-R