In iMovie 10, @soulshined 's answer doesn't work for all clips if you've already dragged your clips into the timeline. It would be onerous to remove all clips from the timeline and then re-add them again if the clips already have other edits applied that you don't want to manually re-apply. To apply the crop mode in-place to all clips you already have on the timeline, do the following:
- Crop (or adjust) just one clip to the desired final alteration
- Select that clip, and then click on the Edit menu, then click "Copy".
- Press Cmd+A to select all other clips, or shift+select just the ones you want
- Click on the Edit menu, then go to "Paste Adjustments", then click "All" (or click "Crop", if you only want to apply the crop adjustment).
Note this doesn't work with normal copy/paste, you have to either use the "Paste Adjustments" menu or a special paste command (which happens to be Cmd+Alt+V on my computer, but may be different on yours)