Hi..
well i didnt understand what you want to do first...aspect ratio is inversely related to overdrive voltage of transistor..u can get their relationship from current equation also....it depends on what u want your transistor to do and to work in particular region....if u want more current u can increase aspect ration but that will affect ur vds sat. so u can either first fix ur current then get vds sat and u can find aspect ration..
there r many ways to do that..also if u know gm and id u can get minimum aspect ratio.
hope that helps

hi watt

a very general rule of thumb is to size all transistors to have a gm/i of ~10.

then for more speed, you size it greater than 10, and use minimum L

for mirrors, use large-ish L, in the order of 1u (on a 0.13um/0.18um process) to get good matching.

for low flicker noise, use large WxL area.

One thing to start thinking is about layout. It looks neater when transistors have uniform dimensions (e.g. use L=1u, 2u, etc ... rather than 1.12u, 2.38u etc ...)

the most crucial part of the design (and most often forgotten) is not the sizing of the devices, but rather the BIASING.

One good book to follow is ...
CMOS: Circuit Design, Layout, and Simulation
R. Jacob Baker

i think he also has some lectures which you can follow online ...

good luck !

Thank you harpoon and jockeymoon, your advices are terrific great. Thanks a lot.

gm/id is also a good way to design, I am a new learner. Maybe I need to train my intuition.

What i would advice is to initially get a feel of how the transistor vgs as well as vds behave in simple circuits. Firstly get hold of bias blocks and try putting the transistors in sat  by modifying the gate bias, have a feel for how you sum up all the voltages from supply and reach ground.Razavi as well as baker are very good texts with baker being more design  oriented.