There is a simple way to change the size of Windows 7 Taskbar Thumbnails. If you find them to be a little to small for your liking, go ahead and make them bigger!
Open Regedit & navigate to the following location:
In the RHS panel, create a DWORD value named MinThumbSizePx.
Right click MinThumbSizePx and select Modify. Set its decimal value to say, 300.
Exit the Registry Editor & Logoff > Logon.
You will now see that the taskbar thumbnail size has increased.
We all know how to increase thumbnail size in Vista, but this method for Windows 7, has been suggested by my MVP colleague Ramesh Srinivasan.
For more, visit Winhelponline.
UPDATE: Our Taskbar Thumbnail Tweaker for Windows 7 may interest you.