I was also facing same issue.
I installed 6.0.8 form here on my windows machine then installed ubuntu-19.04.
When I start it, got small window screen.
I looked for that in many articles such as-
https://askubuntu.com/questions/314685/is-there-a-way-to-make-a-fullscreen-on-virtualbox
https://superuser.com/questions/930184/maximizing-window-for-virtual-machine-in-virtualbox
https://askubuntu.com/questions/22743/how-do-i-install-guest-additions-in-a-virtualbox-vm
Last article advice me to install some Guest Additions from Virtual Box but that also didn’t worked.
solution was very small and easy-
Login to the ubuntu machine.
Look for settings > Devices.
Changed Resolution as 1920 x 1200.
Clicked Apply.
Choose “keep configuration”.
That produced required result.
Note- You may have different regularization size. sometime may not have suitable resolution.
Hope will help you!
If you want to make a VM window to be full screen, you have to install the Guest Additions package in your VM. To do so, you have to:
Once you have launched your VM, in the top menú, select “Devices -> Install Guest Additions”. After that, the Guest Additions .iso will be loaded.
You will see the .iso loaded as a CD image. Run it and it will launch the install process in a Terminal. Follow the instructions until the end of the install process.
Reboot your VM.
After that, you will be able to make the VM window full screen, or any size that you want.