I have changed the maximum thumbnail size, via Preferences / Misc / Advanced to 640 and restarted.
The max size for the Thumbnail slider or it's max size button has not changed.
Can I change the thumbnail slider to use the large thumbnail size?
Note: I am trying to replace Adobe Bridge; do you have an idiots guide
They should both update automatically, as long as you have restarted the program.
Note that if you click "Restart Later", you need to use File > Exit Directory Opus to restart the program. Just closing the window isn't enough (by default).