By default thumbnails are displayed in 96x96 pixels. If you want to increase their size:
- Click on Start > Run > type "regedit"
- Click HKEY_CURRENT_USER > Software > Microsoft > Windows > CurrentVersion >Explorer.
- Click on Edit > New > DWORD Value.
- Rename it as "thumbnailsize"
- Double click on it, select Decimal and enter a custom pixel size of thumbnails (for example 200)
- Click on OK and close the registry.