Fixing page width?

Hello,

I have searched and cannot find an answer and I am not sure where I should post this.

I developed my new website on a medium sized notebook, but now when I view it on wider/larger computer screens I do not like how the entire page will widen to meet the edges of the screen.

How can I fix the width? Can I place a different/new background image for the overflow area? What is a good dimension to restrict the page?

Thank you!