Window Resolution Size Setting

Why does the game only allow you to change resolution when in Exclusive Fullscreen?

I use a 32:9 monitor which is not ideal for fullscreen gaming, therefore I play in windowed.
Ideally, I'd like to know why you can only resize the window with your mouse. It's a great feature but I have to drag the window larger every time I open the game. Lame.

Nevermind that it opens a window, goes to fullscreen, and realises it needs to go back to windowed mode again. I just want it to open at a resolution I set, so I can have clips at a consistent aspect ratio without cropping anything out.