Hello. I was playing with the source code of libfm and pcmanfm and did some improvements in thumbnails handling. First of all I separated thumbnail size from icon size. From now on thumbnail width can be bigger than icon width, but thumbnail height cannot. I did this that way because I didn't want the icons to jump like in Nautilus. But of course this can be changed. I added some options to pcmanfm's preference dialog to change the thumbnails size. Its done in a little different way that selection of icons size, but I think that GtkSpinBox is quite good. The change of thumbnails size is not immediate because it would lead to large number of reloads while changing the value in spinbox. Because of that the changes is delayed by 2.5sec.
The patch was tested with libfm and pcmanfm from git (18 nov 2011) and it works good. I also included the changed pref.ui file just in case.