On the topics of Electromigration You have a description in Your Technology Manuals.
(at least UMC013 and TSMC018 Manuals have a section about the topics)
Or use the Bible ! ;-))
(Hastings: The art of analog layout)
electromigration is one of the topics that is important to understand in IC design. a lot of books and sites that discuss what it is. you can also refer to the friendly wikipedia.
Width of Power Metal line depending on Electro Migration(EM) and IR Drop.
These EM & IR depend on process and technology. The current density and resistance are different for diferent metal.