Can I resize my screen? I have cracked my phone on the top-right corner, which makes me unable to see my notifications. It is such a big issue, I would really like to make my screen smaller, just so I can see them again.

Is there a way (preferable non-root, but if it has to be rooted it's ok) to resize my screen? If you grant WRITE_SECURE_SETTINGS to an app you can do things like enable fullscreen in all apps, so is there maybe a way I can make an app that resizes my screen all the time?
If your device have One-Handed Mode option, then you can use that. One handed mode reduces current display to specific size(say 4inch etc). If your device screen is of 5.5 in then using this mode you can easily pull down notifications (you said that your top-right corner is cracked). 

In Mi devices, One hand mode is activated using navigation key combinations(swipe from home key to left or right). You need to activate it from settings before this. 

In Moto devices, swipe from screen's center to left or right bottom corner.

In case if you don't have this option, try this rootless application from  XDA