The W/L ratio is used in the design of integrated circuit transistors. It affects the gain, capacitance, and current capability of the transistor. If you are doing that type of design, then you need the Spice model parameters for the particular integrated circuit fabrication process you will use to build the IC and then perform Spice simulations of the circuit to optimize the W/L ratios for each transistor.
Decide what your terminal voltages and current requirements are, then use iD=un,p*Cox*W/L(vGS-Vt)^2. Or use the more general equation, which I can't remember at this time.