There are two ways here to increase the thumbnail in windows vista taskbar. Firt either use
Vista Thumbnail Resizer. A freeware tool which you can use to set the size of thumbnails. It is very easy to use. The second step you try to edit the registry editor. Here is the simple process. Run the registry editor and navigate to this location - HKEY_CURRENT_USER\Control Panel\Desktop\WindowMetrics. The click on Edit > New > String Value. Type the name MinWidth and then add the value "-285". Click on OK and exit registry editor. Restart your windows.
Bookmarks