https://en.wikipedia.org/wiki/F2FS#cite_note-15
https://www.mail-archive.com/[email protected]/msg04247.html: "Currently, it doesn't support shrink an image.", why?
Otherwise there's the method:
Back up the data on the F2FS partition.
Delete the partition and recreate it with a smaller size.
Format the new partition with F2FS.
Restore your data from the backup.
But it would take long time if lot of data...