Can I copy my files into that partition?
Yes, use sudo mkdir /var/directory
to create "directory" and then do a sudo chown $USER:$USER /var/directory
and you can move/copy files to /var/directory
. Use it as a temporary method though and not to permanently store files.
What is the minimum space I should leave free under /var?
0; with a partition a few Gb 1, 2, 5 is enough. I doubt it will use up extra space when just using your system. /var will get used when installing new software but to fix your space problem you can just not do that.
A desktop should not have a /var/ partition. For servers there is softwares that use /var to store data (like MySQL) and even those have settings to put the data somewhere else.
My PC is running out of space. I need a temporary solution to store some files.
With that you are 100% certain backup of /data/partition backups up all your data and when reinstalling you format /, /home (if you created that) and just mount /data/partition. When you have a SSD and HDD and you put the system on the SSD and the datapartition on the HDD you can even take out the harddisk, put it into another system and you have all your data available.
I once set my system up like that and have moved to 2 new notebooks (all with 2 harddisk slots) using a new SSD and the same 1Tb HDD.
du -h /var
) showed it at 5gb. the size you need in your directory is dependent on your usage (what is running, apps etc) as shown by your system using double mine. on my current workstation I'd happily use 30gb (if I had it free), but know on one server I'd not borrow any (its role uses /var quite a bit)