start with your desired gain. we know that gain of a diff pair is gm * rout1||rout3 where rout1 is output reisistance of the diff transistor and rout3 is output reisistance of the mirror transistor.
since your W/L ratio can be the same for many different L don't worry about W now - you will seem to solve for it later.
if you want 50V/V gain (Av) you can either have a 50uA/V mosfet, W/L of 1 and Rout of 1,000,000.
or...
50uA/V mosfet, W/L of 100 and Rout of 100,000
or..
100uA/V mosfet, W/L of 50 & Rout of 100k
now you already know a big mosfet is quiet and slower, and a small mosfet is fast and noisy but remember that larger has much better matching which i think is much more important. it is all these tradeoffs that make your final W/L
in fact, most of the designers i know only use minimum L (0.25u) for their digital! they feel that the very small Rout of a 0.25u device gives such bad current accuracy it's no use!