I have not seen a MOS transistor symbol with these features (neither in the PSPICE lib nor in the lib offered by nhatkhai in Adding spice components that do not exist in pspice library). If you look at it from the PCB side, the lack of this feature is understandable, because as a user of discrete devices you will probably never stumble over parameters like W and L as user definable entities.
If we want to use eeschema in a more general fashion, as a front-end to ngspice, this feature would be very desirable.
So the question here is: Can we define a symbol within the current eeschema framework that offers text fields to enter W and L and adds them internally to the device instance?
This then should result in a netlist line like
m1 d g s b mosmodel W=xx L=yy
I don’t have an answer to this. If it is not possible, this request should be placed as a feature request to the list https://bugs.launchpad.net/kicad.