How can I increase the maximal size (when enlarging them with Ctrl +) and resolution of thumbnails in dolphin? If not, is there any other file manager which supports larger thumbnails than dolphin to preview pdf's and djvu's?
This is unfortunately impossible without hacking on dolphin. The maximum thumbnail size seems to be hard-coded. If you e.g. want to rename files quickly based on the preview of the cover pages of the documents you might find the "Information" toolbar useful. It also displays a thumbnail preview whose maximum size is less restricted and you can enhance it by enlarging the toolbar. It allows to quickly scroll through the files and to read the titles on the first page (if they are not too small) without opening them. If this is not satisfying for you, consider creating a feature wish on https://bugs.kde.org .