In iMovie, how to see entire portrait photos?

You would use the crop feature's "Fit" setting.

There will be a black boarder around the portrait photo to make it fit iMovie's 16:0 screen that cannot be changed.

-- Rich

Is there a way to do this when in the app on the iPhone (as opposed to the Mac)?

I’m not too familiar with iPhone iMovie. I think you use the pinch feature (with your fingers) to adjust the size of the photo.

— Rich