The width of a website is the width of the person's moniter who is viewing it. My laptop's width is 1366 pixels whereas my desktop's width is 1024 pixels. It varies a lot.
Don't use the <center> tags as they are an old standard. If you are using CSS, you can center an element using margin: 0 auto;.
You can also design your main website to be a certain width. There are many tools out there to help you, especially the 960 grid system.
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