Adding a youtube movie and changing the size doesn't work, it resets to the standard values of 425x350.
This problem is a bug in TinyMCE I believe.
After some research I found out that the plugin uses TinyMCE 5.3.8 and according to the patch notes of Version 4.0.13:
"Fixed bug where the media plugin would override specified dimensions for url pattern matches. Patch contributed by penc. "
The related bug note:
Could you please update the TinyMCE version? Or fix this problem on an other way? Thanks!