So, appropriate themes (although by now you've found one of them): 'normal' themes that can give you the widest fixed-width results are Fusion, Modularity Lite, Nishita and Retro-fitted. In all of them you can eliminate the sidebar:
Fusion - you select the fixed option and the no-sidebar layout in Appearance > Theme Options.
Modularity Lite - you select the no-sidebar template in the page editor.
Nishita - you select the photoblog layout in Appearance > Theme Options and you don't add any sidebar widgets in Appearance > Widgets.
Retro-fitted - you simply don't add any sidebar widgets in Appearance > Widgets.
The widths you get this way are:
Modularity Lite 950px
Other things to note:
In Modularity Lite and Nishita you can select dark or light color scheme (Appearance > Theme Options).
In Modularity Lite, Nishita and Retro-fitted you can change the background color (Appearance > Background).
In all four you can remove the top menu if you wish.
And I suppose you don't want a comment box on the page. If so, Pages > All Pages: hover over page title, click Quick Edit, uncheck "Allow Comments", click Update.