Export frames in custom size

In implementation of our restaurant software, we are looking for a new and better way to quickly and easily create floorplans. I made a template for this, which works brilliantly. The only issue is that I cannot seem to find a way to export the frame to the size needed. Our program needs a PNG image from 900 x 480 pixels. If larger, it will force the image into the frame and quality is lost even.