Hi,
As Steer has mentioned, the current mismatch can be dramatic for a few mV.
If you observe the Id equation:
=Ido exp[(Vgs-Vth-Voff)/nVt]
Ido=2nβ(Vt)²
try varying the vth by 5 to 20mv and ibserve the change in I
Agreed that as W and L increase matching does increase, but due to high temperature in fabrication process, Vth mismatch can take place and do take place. If by chance you have a metal laid over gate of one of these transistors you wouid surely observe Vth variation between transistors and large sizes would be of no help.
If you need to bias circuit at large distance from the current mirror, and you are doing voltage biasing, you are bound to observe Vth mismatch in transistors immaterial of the size.