Yes it is its a very simple change in a text file. I'll have to find the exact link etc please bare with me.
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The file is called settings.txt in your ~/Documents/Paradox Interactive/Crusader Kings II/ folder.
The line fullScreen=yes controls if the game runs in full screen or windowed mode, for windowed mode change it to fullScreen=no.
size={ x=1920 y=1200} defines the screen resolution. You can edit it manually as well, but it also can be changed in the in-game video options menu.
