hi, suppose one designs a common source amplifier using nfet and a diode connected pfet for a particular gain with all the device parameters such as transconductance, mobility ,oxide capacitance etc are given. Here, by designing the amp, one basically finds out the value of W with L being specific to the technology. So while designing layout we can only set the value of W and L of polysilicon . Then how does one go about setting the dimensions of ndiffision, pdiffusion, nwell, pwell etc? Also when local interconnect from the gate has to be connceted to the analog pin of user wrapper, how much area should be maintained for local interconnect such that the applied signal at the analog pin reaches the input gate of amp without significant attenuation? could anyone give suggestions on this?

There's a lot of ground there. Do you have an advisor you can consult? If not, I found this textbook helpful, even if it is getting a bit dated now: https://books.google.com/books/about/The_Art_of_Analog_Layout.html?id=S3QeAQAAIAAJ&source=kp_book_description

You can't make a (good) common source amplifier with diode connected load. You could make a voltage buffer. The best bet is use a resistor as load, but the really useful amplifier would use an active load. The diffusion areas should be minimal, as allowed by DRC rules. The minimum metal wire width should be enough to let a 1 uA/um², at least. Avoid as much as possible local interconnect for analog designs and route everything in the upper layers, as it is a poor conductor. Try to design a simple single stage symmetrical differential amplifier, then a two-stage miller amplifier. Later, you can try a folded cascode one. The gm-constant self biased current source is a good first example, as it is an essential block for the amplifier. A bandgap voltage reference circuit is a very practical one. Then you could design a LDO, using the bandgap and the amplifier.

@Luis Henrique Rodovalho could you suggest a book for the above?

https://books.google.pt/books/about/Design_of_Analog_CMOS_Integrated_Circuit.html?id=hdavQgAACAAJ&redir_esc=y For analog design in general, Razavi's book is a good one. For analog layout, I don't know any good book. What is more important is to know how to use the tool. You could see other's people designs. They are open source, after all. Just don't look at my designs, because they don't have a decent documentation. Ok, you could look at it as an example of what not to do.