For anyone struggling to find these settings, in BIDS within the Design area, if you click on a blank space of the report you will see the "Body Properties". If you click outside the report area but still within the design area the properties shown will be "Report".
Failing that, in the Properties pane, there is a drop down list directly below it's heading where you can select 'Report'.
For descriptions on how to handle page breaks etc, this is the pertinent BOL entry:
http://msdn.microsoft.com/en-us/library/dd255244.aspx
Essentially, the InteractiveSize Height & Width relate to the HTML renderer. THe PageSize relates to other renderers.