There is no such thing as a "standard size".
You don't need the width of the screen. You need the width of the browser's client area, which is easily handled in CSS.
Google for "
Responsive Web Design[
^]" and start reading.
But, keep in mind that you don't want your site to span the entire width of the browser in all situations. Someone who has their browser full screen on a super-side monitor is going to make your site look really silly if you try to keep your site glued to the sides of the browser. You really should support a maximum width with empty bars on both sides, to keep your site looking nice.