go to you admincp
style and templates
select your template and go to edit
then look for "Main Table Width (pixels or percentage)" and select 800px or whatever you want.
I have my
Blacknight.ie.: Client Centre - Client Login (use the link below....) set to 800px in width, the template i'm using is actually the same template as the one on this forum (default), i just changed the width some css values.
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EDIT: Michele, everytime i put in a link, it gets messed up badly. It could be that i'm using the beta release of Firefox, i'm not sure. It happened before with that "untitled document" link too.