split ring resonator

Do you mean the effective length of a metamaterial sample? Or what?

Effective length was addressed in Chen et al Robust method to retrieve the constitutive effective parameters of metamaterials, PRE, vol 70, 016608. Otherwise I'm not sure what you mean.

if i want to design a rectangular patch using split ring resonator by hfss. the parameters of rectangular patch like length and width is given . how we determine the length , radius of split ring resonator . in camparision to the rectangular patch.

I suggest u look at the Lumped elements model of SRR. That will help I think.

plz check this paper it will very much helpfull to u.