iMovie zoom/ cropping pictures for iPhone 6

Were the photos captured in portrait orientation (vertically)? If so, iMovie can't display the entire image at once and will only allow you to zoom out to fit the width of the screen. As a workaround you can use a panning effect (ken burns) to move the viewable area over time.

1. Tap on the photo in the timeline.
2. In the viewer, ensure Ken Burns is enabled
3. Tap the first position button and adjust the image to how you want it on the first frame
4. Tap the end position button and adjust the image to how you want it on the last frame

iMovie will animate between the two allowing you to see the entire image over time if desired. (Adjusting the length of the clip in the timeline will adjust the duration and speed of the animation.)

CCan you crop the photo first? And if so, what ratio?

You could certainly crop the photos before adding them to your iMovie project, however it likely wouldn't be much different then using the zoom function in iMovie. You would probably want to crop the section you want to 16:9 (widescreen)