I always create swap partition every install new linux distro, and the size usually twice then my RAM around 2 GB to make the system work properly.
But when I was installing debian and used the same swap partition for debian, the swap partition in ubuntu is not available when I checked in system monitor so here what I do to add swap partition in ubuntu after system installation.
This way work for other distro not only in ubuntu, I also try this in arch linux.
For example my swap partition is /dev/sda2
sudo mkswap /dev/sda2
sudo swapon /dev/sda2
To enable this swap partition on boot, add an entry to /etc/fstab
sudo gedit /etc/fstab
then add this entry
/dev/sda2 none swap defaults 0 0
Then my swap available again.



