If you just want the contents, not the browser window (and its toolbars) itself, then Paparazzi might be quite useful. It not only has its own GUI, but also has a URL syntax that supports invocation from any browser, including setting the size:
Turn this into a bookmarklet like so:
In Safari you can assign it to one of the first 9 places in the bookmarks folder, and hence automatically give it something like a Command-1 keyboard shortcut. Note that whatever browser you use, the actual screenshot will always be created using WebKit (just like Safari and Chrome do, but different from Firefox's Gecko).
Paparazzi also supports some AppleScript. Or if you prefer the command line, using webkit2png:
python webkit2png -W 1000 -H 700 http://google.com/
If you also want to include the browser window itself, then I doubt there's a browser-specific tool for that, so you'd have to use your regular screen capture program. To resize the browser window for that, I use the following bookmarklet (which, for some browsers might apply to the inner format, but for Safari and Firefox refers to the outer size):