You can't in browse mode. For the purpose of printing/PDF and in preview mode you can set the portal to Slide Up, Resize enclosing part, to eliminate the unused rows. These are settings on the Inspector's position tab.
There are also report options available that can in many cases present your portal data in a report without using any portal at all on the report layout. Check out how invoices are printed from the Invoices starter solution, to see an example of this.