In the project settings dialog you’ll see a box for stage width, then one for stage height, then a pulldown menu with a list of common sizes. The width and height boxes are what matter — the menu is for faster setting of the size you want.
So if the pulldown says “320×240” but the width box says “1920” and the height box says “1080”, 1920 x 1080 is the size.
Are you using a background image for the layer, or is it only particles? If it’s only particles, I don’t know why it would be using only the center 320 x 240.
Alan.
http://www.wondertouch.com