It involves a registry change. If you are not comfortable changing the
registry, DON'T.
1 Click Start.
2 Click Run.
3 Type in regedit and click OK.
4 In the Registry Editor, click File.
5 Click Export.
6 Enter a name for the backup Registry file, and click Save.
Then:
1 Navigate to:
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer
2 Right click the Explorer folder.
3 Right click in the right side window.
4 Select New, and click on DWORD value.
5 Name the DWORD ThumbnailSize.
6 Right click the ThumbnailSize, and click Modify.
7 Enter a number from 32 to 256 in the Value data box and click OK.
A value greater then 96 will increase the size of the thumbnails. The
change is immediate. You do not need to restart. Keep the registry window
open, and check if the thumbnails are the way you want.