Dielectric resonator antena dimensions

Hi

I want to design DRA on an antenna.

I would like to know if there are some formulas to determine the best dimensions for best efficiency. I only know one rule from some book for now and would like to know if there are more. That rule is that the dimensions of a DRA are on the order of lambda/sqrt(er) or less.

I would like to know if that dimension is for all sides x,y and z and for which shape?
I would also like to know how can I determine what is the best DRA shape for efficiency? spherical, cylindrical, rectangular...
I would also like to know how to determine the best values of a and h in Circularly cylindrical DRAs on a ground plane.

thank you.