MOSFET Sizing in Wide-Swing Current Mirror Design Exercise
Sizing depends on what you want to achieve. You may aim for the specific output resistance, for the specific voltage swing etc. Once you have it, you turn on Monte Carlo to see if the current matching is acceptable.

In your example here you make make L longer for M3 while rather keeping the minimum length of M4. Additionally, M4 will be rather wide, as it is biased from more or less diode-connected voltage, but must still keep the drain of M3 on a useful voltage level.