Here’s a couple of scripts that allow you to change your grid size using your mouse-wheel 🐁.

This is my favorite way to quickly dial different grid sizes since I only have to scroll up if I want more divisions or scroll down if I want less divisions 💪
I will now show you how to set it up for both the arrange view and the MIDI editor.
Arrange view
Import the script:
Copy and paste this link into: Extensions > ReaPack > Import a repository
https://github.com/me2beats/reapack/raw/master/index.xml
Run the script:
You should now have the following script in your Actions menu:
Script: me2beats_Adjust grid (mousewheel).lua

Just set a shortcut to it using your mousewheel. I have it set to Shift + Alt + Mousewheel.
After you set your shortcut, you should be able to use it to change the grid size of your arrange view.
MIDI editor
If you have reapack installed, you should have the following script in your Actions menu in the MIDI editor section:
Script: mpl_Adjust MIDI Editor grid (mousewheel).lua

Add a shortcut to this script. I set mine to Shift+Alt+Mousewheel
After your set your shortcut, you should be able to use it to change the grid size of your MIDI editor.