Altium: can not remove or change black rectangle in panelization
I believe that is the default board set up by Altium. In order to change its size and/or shape, press the "1" key (to switch to board view) and go to Design-> Edit Board Shape as @brhans suggested. You should be able to change the shape from there. Make sure you leave some material around the outside--The manufacturer needs to be able to route out around your boards in order for them to be broken out of the panel. Generally I recommend leaving about 1/2 inch if you can. Check your manufacturer to see what board size they recommend and go from there.