I may have found a solution.
You'll have to input the meta data for movie description in your files.
I found a free Mac app called Subler.
In this app you select you movie file and you can automatically chose to pull plot summary from iTunes once it does it pulls cast and crew info also.
It also will change the "kind" to movie you can then send a copy to iTunes and it will appear in the My Movies Section of iTunes.
Here's is my ripped version of Zero Dark Thirty. In iOS with the full iTunes plot summary.
Here it is in iTunes for Mac.
Now still when doing a option click on a movie in Mac iTunes and choosing get info and then description still yields a cut off description but this is true for iTunes purchased movies.
So I believe any issue is with get info description tab on iTunes Mac OSX
But to get full plot summary use Subler to pull the info from iTunes and add the meta data. It's a free app I found from a Google search not from the Mac App Store. I don't know if there Windows version.
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