I have not found a way to disable the preview feature in ktorrent. Basically I would like to set the preview size to 0, but 16k seems to be the lowest possible value.

Reason is, when downloading torrents with many files, it may take a lot of time to first download all the preview chunks, before it even starts downloading files set to "download first".

Other possibility would be to make "download first" setting priority higher than the preview priority, so the "download first" files would be downloaded before starting to download the preview chunks

You can't disable it