When you have the Aero theme enabled in Windows 7, you see thumbnail previews of the opened windows when hovering over the icons in the taskbar. If you find the thumbnails are too small, you can increase their size by adding the following Registry entry:
DWORD Name: MinThumbSizePx
DWORD Value: (size in pixels)
Maybe start with 350 pixels. You must restart for the change to take effect.