Guest screen size is either large or small with MacBook Pro and external monitor.

[dduck721]

Currently running on Mac OS Mojave and upgraded to VirtualBox 6.0.2 and now 6.0.4 with problem still seen with 2 physical monitors and launching image (CentOs 7). The session opened on MacOS with correct resolution and size once the setting was put to 200% for auto scale output. Now if I move the VirtualBox screen to another monitor which is a higher resolution the VirtualBox screen is very large. Unable to see the entire desktop of the VirtualBox screen. It appears that I can either have the correct size of screen on the MacBook Pro and the 2nd monitor will have a very large desktop -OR- the 2nd monitor will be the correct size and the MacBook Pro has a very small desktop objects which I can't read. Previously was able to move VirtualBox 5.x between the 2 monitors and there was a slight increase in the 2nd monitor but now totally useless.

Have the matching extension package installed.
Have gone into the XXX.vbox file and reset the ScaleFactor to "1".
Have tried setting 2 monitors where MacBook Pro is set to 200% and the 2nd monitor to 100% but 2nd monitor still has large desktop.
Graphics Controller set to VBoxVGA, and have tried the other settings but to no avail.

[cvl]

Having exactly the same problem. Did you get to a solution?

[socratis]

The ScaleFactor isn't error free, it has issues. I've found that setting the VirtualBox Preferences » Display » Scale Factor » Monitor1=200%, Monitor2=100%, Monitor3=100% is my best option. And then sticking to one monitor, not moving it between them.