A question from a newbie to all wise and experienced ones.

My question is regarding how to optimize device W and L in real life design situations. In textbook design problems they generally give values like kp, λ etc. and one can plug these in the ideal square law equations and get W and L. But in real life designs these parameters are not even published in most Electrical manuals of a process. Further one cannot really get it from a plot as you might not take into account short channel effects and so many other factors. Then how is one to optimize ones design for W and L. Should I just use trial and error and hope that I might just hit upon the right W, L combination? I don't feel this is a very good way to design, is it? I guess my question is in real world design situations how should I develop an intuitive feel for the design so I can be in the ballpark of the final W, L and then maybe just slightly tweak things to get the final result.

Thanks