I am currently working on a video (hi-def, 1080x1920, 16x9 screen size). I want to add two different graphic elements at different points in the video (amateur home stuff). The first item is a graphic to be used as a static background picture via a green screen technique. The second item is an animation with an alpha channel to overlay a video scene. Both items will be created using Xara. My question is: How big do I make the document & animation so that they will fit seamlessly into the above video? Do I work with a document size of 16x9 inches or 1080x1920 pixels? I realize this is probably a simple question. But I need you experts to weigh in. Thanks.

Bob