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Thanks for asking.
As this question is coming up again and again, I've decided to now add 
a FAQ entry on this. Please find it below.
The size of the cover display can not be changed currently.
Best regards, 
Felix
--- FAQ ---
Can I also add DivX/AVI/MOV files to the built-in movie library and 
not just VIDEO_TS folders? (FAQ015E0200)
Yes. All you need to do is to install VLC and organize your files in a 
given folder structure. It's also possible to add covers then. If, for 
example, you want to add the file MyMovie.avi, the folder structure 
should look like this:
Title of my movie/VIDEO_TS/MyMovie.avi 
Title of my movie/File with Cover.jpg
You can add movies that are split up into several files the same way. 
The folder structure then looks like this:
Title of my movie/VIDEO_TS/MyMovie part 1.avi 
Title of my movie/VIDEO_TS/MyMovie part 2.avi 
Title of my movie/VIDEO_TS/MyMovie part 3.avi 
Title of my movie/File with Cover.jpg
The names of the movie- and coverfiles are freely choosable - you only 
need to make sure the suffix fits the format of the file. Supported 
formats for cover files include JPEG, PNG and GIF.
In Remote Buddy's menu, you can then start playback of your movies 
complete with existing covers in the context menu of the VLC Behaviour.