I have a rdlc report that conains a tablix, nothing other. The Tablix expands in horizontal direction to show days and in the vertical direction to show groups.
the solution is simple ConsumeContainerWhiteSpace=True (you find this in the property window for "report") you set the paper size in the main menu/ report/report properties for example to "A4" and then you check that paper size is larger then body + header + footer + margins.