Pixel width of Right Sidebar

Hi, can you tell me what the width of the right sidebar is? I am trying to put
an optin box and can’t quite get it to fit right in the sidebar. When I increase
the size…it becomes off centered and doesn’t look right.

Also, what would the size of a footer widget be if I wanted to put a banner there
across the bottom of my site?

Sidebar widget width is 303px but its content are is 273px as there is a padding around it.

Each footer widget width is 306px and its content area is 276px (content area). To add banner across the entire footer, area you might have to edit CSS for those because right now there is 3 seperate columns.