What is a display fusion taskbar ? All I see when going to Settings > Multi-Monitor Taskbar is what you see in this image on this(https://www.displayfusion.com/Help/?Version=7.2.99.104#contextmenu-taskbar-enable) page ?
The DisplayFusion taskbars are on all monitors except for the Primary, which has the Windows taskbar
Right-clicking the DisplayFusion tray icon will give you that menu (Enable/Disable Taskbar), right-clicking in a blank space on the taskbar (i.e. somewhere near the middle of the taskbar) will give you the per-taskbar settings like Size, Position, etc.
7.3 final is available now, and you can resize the titlebar buttons using the "TitleBar Buttons: Icon Scaling" option in the Settings > Advanced Settings window
Sorry, if you search for it with the word TitleBar Buttons in the search, it won't find it, as the option is just called "Icon Scaling" and it's in the TitleBar Buttons group. I've attached a screenshot for reference