The size change works great on Windows 7 taskbar, but I couldn’t figure out how to get DisplayFusion Pro's taskbar (on the second monitor) to reflect the change.
Even better would be an option in DisplayFusion Pro to adjust the thumbnail size for both taskbars.
This is an excellent idea, I'll add the ability for DisplayFusion to be able to detect the taskbar preview window size. Thanks! As for manually controlling the DisplayFusion taskbar's preview size, you can use the Advanced Setting called "Taskbar Preview Width":
http://www.displayfusion.com/AdvancedSettings/