How to resize the thumbnails in the document history?
The task is quite difficult. In order to change the size of the previews in the Undo History, you must find the system-wide gimprc file.
It’s a GIMP configuration file. Go to the following directory: ~\GIMP_foldertc\gimp.0 and open the gimprc file in a text editor. Then you should find the line «# (undo-preview-size large)». Remove the hash sign (#) from it and change the value «large» for the more suitable one.
Possible values are: