[Help] - Where can I hook into the script handling Canvas resizing?

I need to modify some of my GUI based on the size of the canvas. No, I tried using the "relevant" sizing settings already and didn't like the results, especially when I save out my GUI in a wide-screen format and it gets squished when displayed on standard format. I've already addressed the issue by writing some script that can adjust the GUI whenever the player changes the resolution in the options menu. The problem comes with 'Window Mode'. Players can grab the edge of their screen and stretch it any which way they want, creating all kinds of whacky resolutions dynamically. Where would I hook into some script so I can run my custom GUI adjustments even if the player is stretching the Canvas dynamically like that?