Setting the Shop Width?

[touchclothing]

I want to set the shop width so that its the same size no matter how big there browser is. if you look at the site right now http://orange20bikes.com/bikeshop/ the right side move when you stretch the browser. i want the right collum to stay under the header.

[touchclothing]

I found it

#mainWrapper {
background-color: #ffffcc;
text-align: left;
width: 920px;
vertical-align: top;
}