How much disk space should I allocate to "/"?
The bare minimum ammount of space in / in order to run cPanel is 20GB per our installation requirements. The recommended minimum is 40GB.
With that in mind, your server will likely require more space in / based on your usage such as with logs or if MySQL is stored in the root partition.
You might want to review these related articles:
How to use an addition disk with cPanel
How to determine server disk space usage