18. How to change thumbnail size

You can change the size and quality of the shell's thumbnails. Start regedit, expand HKEY_CURRENT_USER (only for you)
or
HKEY_LOCAL_MACHINE (for all the users in this computer) and in Software – Microsoft – Windows – CurrentVersion – Explorer, add or modify the following entries:
• ThumbnailSize – Double Word, decimal value between 32 and 256
• ThumbnailQuality – Double Word, decimal value between 50 and 100