Just like desktop sites, it depends entirely on the resolution of the individual screen. There's no immediate way of determining how wide a given display might be; and the size might even change while the page is being viewed.
It's slightly easier to determine display sizes for cellphones: they're very rarely shown in resizable windows, so if you know what sort of device is making the request (check the $_SERVER['HTTP_USER_AGENT'] string) you might be able to look it up and find out what screen size it uses. But even then, you won't know if the user is holding the phone vertically or horizontally.
Some suggestions for design practice can be found here (when I searched it was the first result that was less than five years old):
http://mobiforge.com/designing/story/effective-design-multiple-screen-sizes
