Hi, tieubanks10,
iMovie 10 has a fixed 16:9 aspect ratio that cannot be changed. When you set the crop adjustment to "Fit" (click the crop tool in the tool bar at the upper right of your screen and select "Fit" from the selection box) a video in 9:16 aspect ratio will display vertically but with black bars on each side of it to make it fit the 16:9 screen. When you export the video it will show smaller because it has been shrunk to fit a wide angle screen in a vertical orientation.
You can work around this by using the rotation rectangles of the crop tool to rotate the vertical video sideways so that it fills the screen. Then export it in the sideways orientation. It will export sideways without black bars. Once the exported movie is on your desktop open it in QuickTimePlayer and use the rotating tool to rotate the video right side up. Then save it. Now the black bars will be eliminated and the video will display larger in a vertical aspect ratio.
Another way to get a 9:16 export without black bars is to use the App Preview feature on your Mac. Go to the projects browser screen (where your projects are displayed as icons). Do File/App Preview. A screen will open that looks like an ordinary project screen, except there are viewer features. import your 9:16 vertical photo. It will display with black bars. However, when you export it the black bars will be gone. To export, chose Preview App from the export menu, rather than exporting it as a file.
A third way to get a 9:16 vid to display without black bars is to export it normally, which will still give you black bars in the export. However, you can crop out the black bars by using the Command-Shift-5 feature. A control box will appear at the bottom of your screen. Select to Record Selected Portion of the video as you play it on your screen. You would begin by drag selecting around the video to eliminate the black bars so that you screen record only the vertical video itself.
-- Rich