Thank you. So do you mean that i should calculate it using Gaussian software. I am still Gaussian beginner, are not there any other procedures to calculate it? and does it depend on solute or solvent or both?

You do not really need high level calculations for the Onsager radius. Just a rough molecular mechanics calculation would do. Then measure the distance between two atoms at the extremes of your molecule and add the atomic radius of those two atoms. Bear in mind that considering a sphere is a very rough approximation, so there's no point in doing ab initio here.

I've attached a publication in which we do this.

Article Photophysics and biological applications of the environment-...