There are css-files for different screen resolution in the [YourMagentoDir] /skin / frontend / default / [YourSkin] / css / folder.
- media-800.css - for screen wider than 800px
- media-480.css - for screen from 480px up to 799px
- media-small.css - for screen less 479px
In order to change desktop site width please open [YourMagentoDir] / skin / frontend / default / [YourSkin] / css / media-800.css and find the following code in the top of the file.
For example, you can change the width of the template to a smaller size:
Fixed width: