When you create a new project in #iMovie, the first thing to do is to import a file that has the quality in which you want to export the video.
Example:
If you want to export a video in FullHD (1080p), a FullHD video sequence must be imported in first place. #imovieexportin1080p
If the first thing you import to your iMovie projet - like a photo, a slide show, etc. - with a lower quality (p.e 720p), iMovie will take this lower quality as referral. Therefore #1080isgrayedout
Tip:
For this reason, I always have a video in FullHD on my desk that I import first (then it is thrown out again).
The individual steps:
00:01 Problem: 1080p grayed out
00:23 Problem solving
01:25 Create a new project ...
... and first import a video in the highest quality, or in the quality in which the export should be,
3. Import all other files, titles, sounds, etc.