The answer is: "the one that fits into my browser window".
Seriously, that's the answer. If it's at all possible (and 99% of the time it is), design a site that will stretch and squash to the available width. That way people with narrow screens won't be visited by the evil that is the horizontal scrollbar and people with wide screens won't have loads of wasted space down the sides.
You would be well advised to add a min-width (maybe 600px) and a max-width (maybe 1200px) to give some boundaries to the design, but within that, just go with the flow and give me the size that I want.