Usually, you do simulations to extract a few things for devices as function of gm/Id. For example Id/W, gm/Cgg, gm/gds, Cds/Cgg, Cdb/Cgg - all as functions of gm/Id. These things are independent (to 1st order) on W, but depend on L. These functions are accessed in Matlab as tables. You can create matlab functions and use them to find for example Id/W for a given gm/Id. Or maybe find gm/Id for given Ft=gm/Cgg, or find Ft for given gm/Id. When you do design you try to work with gm/Id as your primary variable and then find the rest of the design parameters from there.