Hi
The size of the graphic is less important than its aspect ratio which should be 16:9 in order to fill the VP screen fully. If its not 16:9 (i.e 1920 x 1088, 1280 x 720 etc) and it can stand to be distorted then use a third party graphics package to change the ratio to 16:9.
Otherwise add the graphic as it is to the media list. Drag it to the timeline and drop it in position.
Select it and then use the CROP function to trim it but set the aspect ratio to 16:9 first by clicking the corresponding "Keep Proportions" box. labelled 16:9
Drag the sides of the rectangle out to make it as large as possible. It will remain 16:9 ratio. You will probably lose some of the top and bottom of the image when you do this but this is unavoidable unless you distort it as suggested above. But you can position the rectangle to give the most satsifactory effect.
Nat