The actual value of W and L would depend on a bunch of things depending on the application,
The Area i.e. W*L might be critical.
If you want less area and less capacitance, use the minimum L otherwise go for higher L.
If you want less mismatch between devices in a current mirror for example, you would need bigger area and hence larger L.
If you want to reduce the flicker noise which is inversely proportional to area, go for higher L.
If you want higher impedance i.e. ro/gds go for higher L.
If you want good Gm, faster speed, go for lower L.
IT is usually a trade-off between a bunch of things and depends a lot on your requirement.