I may be misunderstanding this post so I post my question...
do you want a single uniform gradient to show through all views an panels or do you want each view to have its own gradient resulting in a non-uniform background?
if you want the second, the answer has been given by LucaS, but if you're looking for a single uniform background, then set the activity background to your liking and set all panels background to be transparent (opacity).
activities will always take full screen, independently of screen size or resolution. a panel can have different sizes and the final size depends on a few factors like screen resolution, used anchors, etc...