How to edit NMOS/PMOS W/L of an inverter layout using SKILL code

Hi

I am using Virtuoso IC6.1.6.

I would like to parameterize an inverter layout for automatic layout design from it's schematic.

Therefore I am trying to edit of NMOS/PMOS W/L of an inverter using skill code in following way:

1. I am using instances (NMOS/PMOS pcells) from technology file (ex. umc, tsmc or gpdk) and made a inverter layout.

2. I want to edit 'width/length' of NMOS/PMOS using skill code

Please let me know how can I write a skill code for the above case.

 I would be glad if you may provide some examples or test case.

Looking forward to see a quick response.

Thanks.