1. Press start and select Run.. button.
  2. Type "Regedit".
  3. Find the key "HKEY_CURRENT_USER \ Software \ Microsoft \ Windows \ CurrentVersion \ Explorer \" and "ThumbnailSize". Doubleclick with left mouse button and enter a value between 32 and 256.

If "ThumbnailSize" is missing, right click on an empty area in the right pane and create a DWORD with the value "ThumbnailSize". Double-click"ThumbnailSize". Choose decimal in the Base section. Enter a value between 32 and 256.