Watching a VSE demo using the Gooseberry branch, the Backdrop can be resized and moved using V. Is there a way to resize and move the Backdrop in the 2.73 build?
The move feature may not be in the 2.73 release. Goosebery is a much more recent development version of blender.
I hope that this feature will make it in the regular release, as it is very useful, especially in full screen mode.