Newbie here: How to size a Web page mock-up in PS?

I've heard a lot about designing web page mock-ups in Photoshop, and then transferring them to a program like Dreamweaver (which has the "Tracing" function). I have a couple of questions about this--if anyone can help, that would be great:

--Does the PS mockup have to fit the actual Web page measurements exactly? If so, how is this done?

--Also, in general--how does one know exactly what size to design a navigation bar or banner in PS so that it's the right size once you insert it into a table in a Web design program (like FrontPage or Dreamweaver)?

Thanks!

I've heard a lot about designing web page mock-ups in Photoshop, and then transferring them to a program like Dreamweaver (which has the "Tracing" function). I have a couple of questions about this--if anyone can help, that would be great:

--Does the PS mockup have to fit the actual Web page measurements exactly? If so, how is this done?

--Also, in general--how does one know exactly what size to design a navigation bar or banner in PS so that it's the right size once you insert it into a table in a Web design program (like FrontPage or Dreamweaver)?

Thanks!