There is a simple way to change the size of Windows 10/8/7 Taskbar Thumbnails. If you find them to be a little too 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 may interest you.