Why do you need W/L of the power FET.
Generally they only mention RON (ON resistance) of the switch in the data sheets. I dont think you will know unless you are a designer. You can calculate W/L approximately if you know Un && Cox and supply voltages. you can use below eqn to calculate W/L. this should give close value.
RON=1/((Un*cox*(VGS-vth)*(W/L)).
For different technologies it will be different. It also depends whether it is a high voltage switch or low voltgae switch. your question is not clear.
Let's say for 0.5um CMOS process, if you want an 60V switch with 1A capability, W will be in thousands and L is approximately 1.5 - 2.0 (L is bigger because of HV).