Hi, Good day!

Can someone explain to me how to solve for the aspect ratio (W/L) of each of the mos transistors of a supply-independent current mirror circuit? I was planning on operating the cm circuit in subthreshold region. Help me please.  :'(

P.S. Iout = 5nA, Vdd = 0.5V - 1.8V (just choose what's best for the circuit)