Output Resistance - Small-Signal MOSFET
Vgs(th) =Vt has a wide Mfg. and temperature tolerance    50%? Measured at some arbitrary low std current threshold.


yet 1-Vgs/Vt= k controls both Ron or RdsOn =1/gm.  
source output impedance = Ron +Rd =Rout here
RdsOn is usually rated around k = 2.5 to 5. Or both where the difference is small.
thus for Vt = 2 to 4V , Ron might be rated at 10V ( these are all the old enh FETs )
newer Fets with lower Vt were designed for “Logic level” thus rated at 3V or 5V for Vgs and Ron
there is also a strong temperature shift included in this tolerance of Vt.


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So you get the idea , that computing it is not so easy but you expect Ron to be &lt;= Rd in a linear design and &lt; 2% of Rd for an efficient switch.  
Yet I have given you hints to solve Rout
but if you make Ron &lt;&lt; Rd then you must be aware that Coss * Ron is somewhat constant in a given design so that can affect turn off time.
So choose your FET Ron wisely, otherwise you need your thermodynamic skills to design the heatsink