Windows 10 have a thumbnail preview every time you hover the pointer above a running program on Taskbar. The default size of the thumbnail is about 200 pixels. However, you can modify the number.
In the following guide, you will learn how to resize Taskbar thumbnail preview on Windows 10 (and may also Windows 8 and Windows 7).
1. Find and run regedit via Start Menu.
2. Copy and paste the following directory into the address bar, then press Enter.
3. Right-click on the blank area of the right panel, hover to New and select DWORD (32-bit) Value.
4. Rename the new key to MinThumbSizePx and press Enter to confirm.
5. Now, right-click on the MinThumbSizePx and select Modify…
6. Enter the desired thumbnail resolution (in pixel) on the Value data column. Then, hit OK.
7. Restart the computer to take the effect.
It also still works perfectly in multiple windows configuration.
If you changed your mind, you can always revert to default by deleting MinThumbSizePx, and then restart the computer.