Just to give you another data point: my Kubuntu 21.04 installation (upgraded continuously since 20.04 or earlier) has a 100GB / partition, 53GB of them used. This is for mixed home/office/photo/science use. The biggest contributions are:
/var: 7GB (of which 5GB correspond to flatpak)
/opt: 10GB (5.5GB for COMSOL and 3GB for conda/python)
/srv: 17GB (14GB for VirtualBox and 3GB for Steam)
I will say something obvious, but nevertheless: how big / needs to be will depend a lot of what you use the computer for, and the location you put things. Until very recently I had a 30GB root partition that was chronically almost full, and all the stuff now on /opt and /srv was in my data partition. When I changed the drive on the notebook I decided to give / enough room to never worry about space again, and moved there everything that was a ‘program’, even if it was not the installer’s default location (COMSOL, conda, Steam, and VirtualBox would all install in a /home/username subfolder if not installed as root).
If you have room, for your use case (light office use and photography) I would give it at least 30GB just for the peace of mind (‘almost full’ partitions are not good in general, and specially if it’s constantly written into).
Read more here: Source link