Okay, you need to go to File -> Preferences and then head to the Test Document Tab. There will be a box for "Run Game In A Window."
I don't believe however, you have any control over the default launch size of the window. You just need to quickly grab the corner and drag the window larger.
Edit: Seems like it retains the window size that you drag it to, even after closing the game and relaunching from the editor.