Most Linux distros only use a couple of GBs for the OS, but if you plan on
adding lots of applications or fonts then you will need more. If you run VMs you
should change the VM directory to /home/user so that it does not use the root
directory. Most of the time 4 GBs will suffice even if you install lots.

My / is 8 GBs and I still have 5 left after installing lots of programs.