For a system using the standard apt package manager, I'd usually recommend to clean the apt package cache by any unused versions of downloaded packages or completely, which will often free a few hundred megabytes.
In your case, your disk utilisation suggests that Ubuntu's proprietary snap packet manager would claim about 270 MB for caching.
I'm not familiar with snap, but I assume that it would also offer a way clearing its cache.
You should consider Ubuntu's support forums for details.
Note that this option doesn't control file size directly - it rather defines the maximum retention period that Pi-hole will store in its database.
If your Pi-hole would process a million evenly distributed queries in 28 days, the file would grow large enough to hold about 500,000 entries for your 14 days.
And if your Pi-hole would process those million queries in an hour, the database size would still grow proportionally for the full amount.
So while your option will result in the database holding only the data for the last 14 days (and thus may well prevent a fresh database file to not grow over a certain limit), it won't change the size of a database file that has grown to a certain size already.
You could instruct SQLite3 reduce its database file size by using the following commands:
sudo systemctl stop pihole-FTL.service
sudo sqlite3 /etc/pihole/pihole-FTL.db "VACUUM;"
sudo systemctl start pihole-FTL.service