Have you tried adding ui_resolutions=hd to your manifest? You should be able to just design for HD and let the firmware handle scaling for other resolutions.
If I don’t set a size to Poster (in the tag, or programmatically), the image is full resolution, off the screen. I’ve tried variouloadDisplayMode settings, too.
If you are specifying sd,hd,fhd then you’ll have to handle all the resolutions yourself. It’s much easier to simply specify hd and treat every screen as 1280x720 for setup and let roku resize it for you automagically.
You must always have a width and height included to use to loadDisplayMode properly.
What you may want to do is something similar to this:
loadDisplayMode="scaleToFit"
width="1280" 'the actual width
height="480" 'the actual width
loadWidth="690" 'the actual width you want to load
loadHeight="240" 'the actual height you want to load