I am currently using PCManFM, but I believe that other file managers are the same. When I open up the directory, large files, that are over the specified size limit are not shown as thumbnails, but only as the type-icons. I work on the directory of many large sized PNG files, and what I've noticed, when I open one of them with Gimp, it does something and from then on this file is shown with the thumbnail unless I rename/move that file.
It would be very useful to be able to "nudge" the specific directory to generate the thumbnails regardless of the file size. Is there a way to script whatever it is that Gimp is doing, perhaps using "imagemagic" to generate the thumbnails and place them in the correct place somewhere.
Just set the thumbnail size limit to 0. Setting the size to 0 makes it ignore size.