Id1 is constant ( the MOS is in saturation because Vgs=Vds ) and Id2 is added to this current so R=Va/(Id1+Id2)
Id2 can be changed by Vgs voltage ( Vc ) and W/L ratio for M2 is easily found by applying inverse Id equation.There may be many W/L combination but starting with min. L will give you an aspect. Then you should consider max. allowable current per um^2 defined by your process and you play around w and L to get optimum solution.( Matching,Process sensitivity,noise,temp. effects etc)