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