How to increase /tmp folder size manually

You don't have to do all that, you can just mount /tmp as tmpfs by using a line like the following in /etc/fstab:

tmpfs /tmp tmpfs mode=1777,nosuid,nodev 0 0

You can also do it live (but bear in mind stuff that is currently in /tmp on your current filesystem will not be able to be accessed except through the inode and currently open file descriptors, so you should really do this by modifying /etc/fstab and rebooting):

mount -o mode=1777,nosuid,nodev -t tmpfs tmpfs /tmp

Note that a tmpfs can swap. If you want a truly RAM-only filesystem, use ramfs.

I figured I could just test it, so I ran:

sudo mount -o remount,size=2800M /run

Worked like a charm:

Filesystem      Size  Used Avail Use% Mounted on
tmpfs           2.8G   45M  2.7G   2% /run

So I filled it a bit:

fallocate -l 1G /run/test.img
fallocate -l 1G /run/test2.img
fallocate -l 500M /run/test3.img

Result:

Filesystem      Size  Used Avail Use% Mounted on
tmpfs           2.8G  2.6G  208M  93% /run

System is still up and running. Swap availability dropped, which proves it was used:

• 17:10: create 2.5 GB of files in /run
• 17:20: remove the 500M file

Total swap is reduced by the amount taken by /run.

I'd test 10GB on a VM, because I don't know if the kernel will refuse the remount or just have an unexpected behavior.

I'm still looking for an actual answer, but the pragmatic way showed it works.