I use Screenflow on Mac for recording screencast. Most of these recordings are done at 1200x720. Screenflow records the entire screen then you later crop it. The problem is you don't know before hand what will fit into the cropped area.
Is there any tool available that can draw a black box that just a border? It should be transparent inside. This would allow me to position the box and know that everything within it will fit into the recording.