Open Registry and navigate to "HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer". Click on "edit->New->DWORD-Value" Name the new entry "ThumbnailSize". Double Click on the entry, select the "Decimal" radio button and enter the number between 32 and 256. The default value is 96. Enter larger value to increase the thumbnail size. Confirm the change by clicking on "OK". Restart windows Explorer for the changes to take effect.