There is a simple way to change the size of Windows 7/8/10 Taskbar Thumbnails. If you find them to be a little to small for your liking, go ahead and make them bigger!

Increase taskbar thumbnail preview size

Open Regedit & navigate to the following location:

HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\Taskband

In the right hand 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 Windows Vista, but if that method does not work, then you may try this method which has been by my MVP colleague Ramesh Srinivasan.

UPDATE: Our Taskbar Thumbnail Tweaker for Windows 7 may interest you.