Well, seeing as no-one else has replied...
In CK there's no easy way to change resolution - it's fixed at 1024x768, which is why it appears in a window. Your monitor may have a setting to stretch it to fit though.
Alternatively if you look through the mods forum you'll find instructions on how to hex edit the .exe to get different resolutions. This didn't work for me though (and I'm fairly sure I did the hex edits correctly).
Finally you can buy the Deus Vult expansion - which has a settings.txt file which you can edit to get any resolution that you want. That's what I'd recommend and the games looks a million times better than it did when it was at 1024x768 and stretched to fit on my screen.
Have fun
Finn