If you made a separate /home partition, then it should be easy to keep your data through any migration, upgrade, downgrade or re-installation. Just choose the manual partitioning and make sure that /home is not formatted.
If you do not have a /home partition, then it might be time to think about adding one, but you will need to backup your data first and then restore from the backup.
If you are wondering about how big to make the root (/) partition, you can look at the overall current size with df
and du and use that for your guess.
(As usual, backup your data)
for i in *;do du -sh "$i";done