I would suggest using SHARE/EXPORT MOVIE and choose 1920x1080 (if your source material supports this).
That should work well in YouTube.
If you want to go further with custom QuickTime settings, here are some that should work, but in my experience, it will be no better than using SHARE/EXPORT MOVIE.
follow the instructions below to create a .mov video that will work on YouTube:
- In iMovie, select Share > Export Using QuickTime
- Compression Type: H.264
- Frame Rate: 30 is preferred. 23.98, 24, 25, 29.97 are also acceptable.
- Data Rate: Automatic
- Key Frames: Automatic
- Frame Reordering: Unchecked
- Format: AAC
- Click Show advanced settings and choose Constant Bit Rate as encoding strategy
- Size: Choose the original size of the video
- “Prepare for Internet Streaming”: Fast Start
File size and quality are a trade off. You can reduce file size in several ways, but they all impact quality.
1) Use a smaller frame size (e.g. rather than 1920x1080, use 1280x720 or 960x540)
2) Lower the data rate
3) Increase the number of frames between key frames
4) Lower the frame rate (say from 30 to 15)
Having said that, unless your internet connection is really slow, I would stick with the bext quality.