Are you trying to design a loop with a natural resonance at 2.45 GHz or a circuit tuned resonance? The easiest way is to create two circles, one larger than the other and subtract. The cut a gap for a lumped port .
Once you create the loop in HFSS, simply look at the real and imaginary Z of the antenna. This is essentially your complex radiation resistance. If you want a natural resonance loop at 2.45GHZ, then the imaginary part of Z needs to be zero at 2.45GHz. You can optimize on this criteria. Also, the radius of the loop will depend upon the domain of your loop, ie constant current, non-uniform current, and Gain requirements.