If you're not married to the current dashboard, you can try the OG Home Page. The newer dashboards have a width control that isn't as controllable for what you're trying to do - and setting exact px widths or 100% only fits the content of that column but the dashboard decides how wide it wants the column to be regardless of what you set. If you use a Home Page though - the same report and setting it to 100% like you have will likely result in the setup you want given it does try and respect the underlying HTML tag when it renders the report.
