Suffusion: 'Header Widgets'

I am trying to change the width of the widget space on the Header Widget (sidebar #12) of the Suffusion theme. The only adjustment I need to the theme is to make the widget wider as the object we wish to insert is about 750 pix wide and currently most of the object floats outside of the wrapper due to the widgets smaller width space.

I tried working with the stylesheet however that did not seem to cause any changes. Anyone have advice on where to look?