Q:

19.04 / Virtual Box 6.08: manager screen fonts huge

This is not an issue with the guest screen resolution, this is an issue with the manager main window opening on Ubuntu 19.04 MATE screen.
It is sized for what looks to be 640x480 -- on a 1920x1080 screen. 
The Settings/Display dialog offers options for only the client displays.
The only likely setting I've found is in the ~/.config/VirtualBox/VirtualBox.xml file :
  <ExtraDataItem name="GUI/LastWindowPosition" value="0,42,960,479,max"/>

But trying a number of different values for 'value', including blanks, has no effect. 
It is interesting that 'Restoring' the window from Maximized before exit, removes the max value from the setting, so perhaps this setting is about the Manager window.  
Deleting that file between invocations of VB has no effect. 
There is a virtualbox file in /etc/default but there is no reference to display attributes in my version of that file. 
I have posted this to the VirtualBox forum at Oracle.
https://forums.virtualbox.org/viewtopic.php?f=7&t=93294
Resolved through 19.04 MATE and QT : oversized display, low resolution

A:

This turns out to be a QT issue resolved by setting this environment variable:
QT_SCALE_FACTOR=1

Put it to .bashrc and all is well.

