Version: (using KDE 4.4.1) Installed from: openSUSE RPMs In Settings->Configure Dolphin->General->Previews, there is a setting labelled "Maximum file size". I assume this refers to the largest file for which previews will be generated. Currently size is determined by a slider which goes from 5 MB to 100 MB. These are very arbitrary limits. I have, for example, a directory with very large (~300 MB) PDF files and want to see previews of them, but Dolphin won't let me. It takes Dolphin only milliseconds to generate previews of 100 MB PDF files, so I'm sure it wouldn't take much longer to do previews of my 300 MB PDF files. Please remove the arbitrary upper limit on the maximum file size and trust the user will be able to set it sensibly in accordance with his or her needs.
Thanks for the report! This is related to (or even a duplicate of) bug 161862.
Problem longer exists in KDE 4.6.3; the Previews tab now contains the following spinners: Do not create previews for Local files above: x MB Remote files above: y MB