Cannot resize screen for Ubuntu VM in VirtualBox

There were two things that were needed: (1) Virtualbox Preferences and (2) Guest Additions for the specific VM.

1) In the top toolbar in OSX, go to VirtualBox -> Preferences -> Display and change your max resolution size. My resolution was set to Automatic and the settings were too low. I changed this to Hint and it allowed me to specify my own Height and Width

2) Start up the VM and log in if necessary. While it's running, click Devices -> Insert Guest Additions CD Image... from the Mac toolbar. There should be some prompts on the VM's terminal then restart the VM when it's completed.

I had similar issue with Ubuntu 18.04. Host was Mac OS Catalina and all virtual box's extensions were installed but to solve the issue I've just selected other graphics controller. To do this open settings -> display and try swith to another graphics controller.