Q : How do you set the game window resolution in the editor to always be 450x600 px ?
Just below the 'game' tab in the editor, there is a dropdown button for the resolution. If you want something that is not on the default list, click the '+' button at the bottom of the list. Select 'fixed resolution' if you want the editor to always run the game in eg/ a 450x600 px window. Select 'aspect ratio' if you want the editor to always run the game in a window with fixed proportion, eg/ 3:4, but you still want to be able to resize the window, eg/ make it 450x600 px or 600x800 px.
Q : Why is the checkbox not displayed in the editor, next to some scripts ?
The checkbox for disabling a Script (in the editor) is only present when you have at least one of :
Start(), Update(), FixedUpdate(), or OnGUI(). If none of these
functions are present, the checkbox is not displayed.
I've read somewhere that if none of these functions are present, the script cannot receice an 'OnDestroy()' event. So sometimes you might need to add an empty 'Start()'.