How wide should a page be?

The easiest and most correct answer here is It should fit. It is actually that simple.

But how do you know your site will fit on the intended media? Will it fit to all screen resolutions? All browsers and windows sizes?

If you want a fixed with, make sure its no grater than 728 px (as wide as a standard ad leaderboard). Then it will fit nicely to a 800 x 600 screen and grater.

But with all the powers you have with CSS; there is no real need for using fixed widths. Try designing your site with percentages instead of pixels. This way the site will automatically fit on screens. Add left and right margins and a minimum width. Then you got a nice basis for a fully scaleable page.

Code example

#pagelayout {
margin: 10% 15% 5% 15%;
width: 100%;
min-width: 300px;
}

If you write good, semantic mark-up combined with CSS you should be able to easily write stylesheets that will fit to any screen resolutions, windows size and device.

Try using the Opera Web browser to see how the site looks on regular screens, handheld units with SSR, projectors/TV and other media.

A great tip for creating a scalable site is to drop site-wide images.