That’s a very open ended question. But I’d say as a starting point you can decide what current you want passing through the transistor and what overdive voltage is required (Vgs – Vt) --- where Vgs is the gate-source voltage and Vt is the threshold voltage.
Use the equation
IDS = K’(W/L)*(Vgs – Vt)^2
Where K’ is process parameters.
I suppose Im showing the formula here in the hope you will find this online for a better explanation
Start with the desired outcome(s) and work backward in
simulation or calculation. You don't have a desired outcome.
So any direction is as good as another.