In the CMOS Op-Amp Design, it often mention that ensure MOS transistors operate far away from the subthreshold region,describe to operate the transistors wiht a minimum gate overdrive voltage.
Vgs - Vt = sqrt(I*L/uCox*2W) >= Voverdrive,min

but when the current is very low,base this equ ,the W/L would be too large.
so i want to know when it's low current ,how to fix the W/L?

i'm so sorry to make a stupid mistake. Actually i wanna to say the W/L would be very very small.i.e. i=5uA,and the Vgs-Vth=0.2v in 0.6um CMOS process

Thanks for your answer!
but the MOS transistors turn into sub-threshold region,i/(W/L)shold be less than about 100nA.then the w/l would large enough,in fact too large i think.

how to fix the sizing when the current is small have been haunting me for a long time.

I should correct that:no the bias current 100nA,but the current of per W/L is 100nA