MOSFET simulation model
It won't really help you but: It depends on the MOSFET you want to simulate: different MOSFETs can have different parameters. Pick the datasheet of the MOSFET you plan to use for your circuit.

The RDSon is proportional to the length and inversely proportional to the width: it varies as L/W.

Here you can find the equation: https://ecee.colorado.edu/~bart/book/ex012.htm