I believe when MS client goes full screen it changes the resolution of your monitor, thus maybe it is not possible make this change because fullscreen windowed render both desktop and game. Fullscreen windowed mode keeps the same resolution it only send its output to the window manager (windows explorer) which then manages where on the screen that output is drawn. This takes some additional performance. The performance penalty, however, is greatly reduced with recent operating system and hardware. It also explain why the current windowed mode is this small.
You can try and use this: http://www.joerg-rosenthal.com/en/resize/ Not using it alot but it does what you want ^^
It's just not possible to do given that we are using a client not coded by the staff. It's essentially a client made by nexon and modified by the staff in order to allow you to connect to Royals instead of GMS. The only way this would work is if someone created their own client, which to my knowledge no one has ever done before. (I do believe that someone on ragezone tried to create their own before but it was scrapped)
