If I have an image in Shotwell that is smaller than my screen's resolution, I would like it to be stretched to fill the screen when I am viewing images in fullscreen mode, like it is in Eye of GNOME and in damn near every other app with a fullscreen mode I've ever used. But there doesn't seem to be an option to do this in Shotwell.
You appear to be correct - no automatic way to do this. Perhaps contact the developers for this option (contact page on their website)
The best you can do is to manually zoom in when in full-screen mode i.e.