Finding the IDs/Classes to change simple page elements

Hi, I'm trying to change pretty simple stuff  by using the adding custom css feature, but I can't figure out what the Class or ID I need to create a CSS for to change the Home page links on the menu bar and the middle bar's style. I also wanted to know if there's a way to insert a bigger image for the header image or if it has to be 300 by 125?   Am I right? Will adding custom CSS be able to change these elements?   Thanks for your help!