The size for the mock up doesn't really matter. It's really just a template for a developer to model after in CSS. The best way to code the side wouldn't be using exact pixel measurements anyway... You would won't the content to appear based on a percentage of space it takes up on the screen or in a specific ares, so that the site remains the same no matter what the resolution of the device it's viewed on..

I would not advise using dreamweaver, I find that software terrible. I advise just coding it from scratch with notepad++. You will get a lot further.

Well, most sites are made up of images that are repeated on a certain axis as a background of a div thus no size is too big or small because it just gets repeated so it stretches.

Most newer sites use percentages as to accommodate for all screen sizes. If you want it to be suited for mobile too, I'd do what facebook does. Facebook uses PHP (I think) to suss out what device the user is on...
If it's a mobile they redirect to m.facebook.com, the m standing for mobile. This site is exactly the same in terms of functionality but is coded a bit differently so that it fits on a mobile device well. The reason they do this is probably because instead of spending ages doing certain CSS to try fit together with pc screen size elements they can just make 2 different sites that work the same and all connected but the UI is different.