You could try 1920x1080 instead. Does iMovie read ProRes 422? If not, can you work in iMovie Pro, aka FCPX?
How should I export my animation out of AE to iMovie,
Thanking you in advance,
Make your composition 1920x1080
Render as ProRes or render as QuickTime with the PNG codec
It would probably do you good to do a little research on video formats and compression. Things work best if you stick to standard formats, frame sizes and frame rates. HD is the standard. 1920 X 1080 @ 29.97 fps for NTSC (most of the world) or 25 fps PAL (Europe and Australia - anywhere that has 50 HZ electricity instead of 60 HZ).
And another thing.....
Unless you're using an old version of AE, you're probably on Creative cloud. That means you have access to Premiere Pro, which is a LOT better than iMovie. I think you ought to wean yourself off the consumer editor and go with the more versatile one.