Imovie in full screen does not fit to the screen (it goes way beyond the screen on the right side)

It looks like you just need to put your cursor in the lower left corner and drag your screen to fit. If you have it dragged two big you will expand if off the screen like this:

When you adjust the screen size by dragging the corner you will get the proper screen size like this:

What you are displaying is not full screen. You get full screen when you click on View/Play Full Screen, and that will make the preview image fill up the entire screen. Nor is what you are displaying the 16:9 widescreen dimension.

-- Rich