You can increase the size of the thumbnail images in Windows Explorer's thumbnail view by using Microsoft's TweakUI Utility or by changing the Registry settings. To use TweakUI, simply run the program and open the Explorer item in the tree at the right and select Thumbnails. If you do not have TweakUI, simply follow these steps:
1. Click on the Start button and select Run 2. Type in 'regedit and press OK 3. Look for this key: HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer 4. In the right pane right-click on the 'ThumbnailSize' key and select 'Modify' 5. You can set the Value between 32 and 256, the higher the number you choose, the larger you thumbnail will be 6. Now right-click on the 'ThumbnailQuality' key and select 'Modify' 7. You can set this value between 32 and 100, with a higher number making the thumbnail a better quality
If the ThumbnailSize and ThumbnailQuality keys do not exist, you can create them by right-clicking on an empty space in the right pane and selecting 'New->DWORD Value'.
NOTE: If you make your thumbnails bigger and of better quality, they will require more disk space and may cause your folders to load more slowly.