You are not missing any option. 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. 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. You can do the same procedure with a 3:4 video.
-- Rich