Your lossless output file is perfectly match to the production pipeline. I don't really use iMovie but I would guess that it would either have to cache the movie to play it back smoothly or there is no cacheing feature because it is a very limited editing program.
Lossless means just that, technically lossless. You want to maintain the best quality you can until you render for final output. You might want to try a test render by setting up an output module template using QuickTime JPEG or QuickTime Jpeg 2000 with quality set to 90% or higher. These files will more likely playback in IMovie.
To match the frame size you just need to match the format on the entire project. As for Rendering for YouTube, and again, I do not use iMovie, but the Adobe Media Encoder does a fine job using the default H.264 YouTube Presets. Just make sure you match your frame size and frame rate from the original project.
I would suggest you spend some time reading up on Rendering in the Help files. Just type Rendering in the Search After Effects Help field at the top right corner of AE and follow some of the links. The third video in this getting started series will give you more of the basics of rendering and delivery. Basic Workflow
Thank you. Hoping for input from someone who uses iMovie. With experience exporting from After Effects into iMovie.
Rendering is rendering. iMovie will playback HD Quicktime Jpeg Compressed movies just fine on my system. Just gave it a test. You probably will not find many folks that use iMovie on this forum. If you do not know how to set up an Output Module, read the help files.
Okay. Could I ask what editing program you use?