how to design a LC VCO?

I need to design a 2.5 GHz VCO using 0.18 um technology. I am thinking about using the LC VCO with cross NMOS pair topology. However ,I don't know what to start with device parameters. How do I pick the value resistor Rp to go with the inductor L? once I get to know Rp, I need to calculate transistor size to get the negative Gm to cancel it out right?

If I use cross coupled PMOS pair on top, I don't need Rp any more? Then in this case, how do I determine the device parameters for the NMOS and PMOS pairs?

thanks a million