The default screen resolution for Core Linux on VirtualBox is 1024×768 and does not provide a good screen area usage.
I came across this post on the Core Linux forum that provides steps to set the correct screen resolution. I am capturing the steps in this post for my reference, all credits are due to the original poster.
I found 1440×762 to be a better resolution if you are going to use VirtualBox in window mode, while 1440×900 is better suited for full-screen mode. In the following command “linux” is the name of my VM.
VBoxManage setextradata "linux" "VBoxInternal2/UgaHorizontalResolution" 1440 VBoxManage setextradata "linux" "VBoxInternal2/UgaVerticalResolution" 762 VBoxManage setextradata "linux" "CustomVideoMode1" 1440x762x24 VBoxManage setextradata "linux" "GUI/CustomVideoMode1" 1440x762x24
Once done you need to update the .xsession file in your home directory with the proper resolution
/usr/local/bin/Xvesa -br -screen 1440x762x24 ...
Now reboot the VM to activate the new resolution or you can exit to the prompt and restart the Window Manager with “startx” command
Here is the result after the configuration