First you must decide for which screen resolution do you want to develop, or you can develop for two or three different resolutions giving the user the option to choose their resolution. However after this decision you must remmember the users from other plataforms like Mac and browsers like Firefox and Safari. Believe me, it´s important to develop your application to all those users. So how can you avoid your page from looking crappy in some browser or platform?

At Webmonkey you will find all these answers. In there you will find tables that will give you the safest size to develop in different browsers and in each different resolution. Now you only have to choose.

Good luck.