Anyone else having the issue where combining classic shell with display fusion on Windows 10 results in the start button on all Display Fusion taskbars being about 20% larger than the one on the "real" taskbar? (I'm not using a custom icon)

And, more importantly, anyone know if it's something I can change the setting of, or A bug BF need to deal with?

Yes, I see the same thing.

The size of the start button in Windows 10 DisplayFusion taskbars appears to be the same size as the start button we have in Windows 8.1 (both Windows primary and DisplayFusion secondary taskbars). The new default start button on the Windows 10 primary monitor is smaller; it appears Microsoft reduced it's size for Windows 10 or some scaling factor is changing it.

So it appears that DisplayFusion is using the larger size start button from Windows 8.1 in Windows 10. Definitely a minor bug.

That's great, the icons are now the correct size, although now classic shells start menu only opens on the real start button.
If you click the display fusion start buttons you get the windows 10 start menu still.

any ideas?

I have the same problem, that is, clicking the DisplayFusion taskbar start button brings up the standard Windows 10 start menu rather than the Classic Shell start menu. Here's some debug files.

Clicked the DisplayFusion taskbar start button at approx. 1:04:25 PM local time.

Classic Shell "Replace Start Button" is NOT checked so it is disabled.

Went back and checked the "Replace Start Button' and this does result in the Classic Shell start menu being displayed. This performance is different than the default performance when using Windows 8.1 / Display Fusion / Classic Shell. In that case, the Classic Shell start menu is always displayed irrespective of the setting of the "Replace Start Button" option. For me, it doesn't make any sense to have different start menus on different monitors.

If possible, would like to see this option set to be the same as Win 8.1 / DP / Classic Shell. No matter what the setting, the Classic Shell start menu should always be used on all monitors.

I also confirm this with DF 7.3 beta 4 & Classic Shell (default start button, no replacements) under W10.

I also noted that pressing the Windows key on the keyboard does display the correct Classic Shell Start Menu. However, it is displayed on the DF taskbar (secondary) rather than Windows taskbar (primary) and I'm not sure if this is wrong or if DF is displaying the Start Menu on the left-hand monitor by default. I cannot see any settings in this regard. But, it is what I would want though. I'm an idiot.. that Start Menu appears on what ever display is in focus.

Appears fixed under the newly released 7.3 beta 6.

Beta 6 appears to have resolved it for me as well, (at least at home it does, not checked at work)