Version: (using KDE KDE 3.5.5)
Installed from: Debian testing/unstable Packages
If I simply did not find the correct option then feel free to close this bug and cuss at me :)
There is no option which would allow the user to set a max size for the thumbnail cache. Just yesterday, I freed more than 500 MB of more or less wasted space.
Also, it would be nice if there was an option to delete thumbnails depending on either their creation time or their last used time.
In light of the impeding KDE4 release, I am going through all bug reports I am involved in. To the best of my knowledge, this issue is still open.
*** This bug has been marked as a duplicate of bug 79943 ***