Resolution doesn't change when resizing Virtualbox window

I had this problem as well running a newly installed version of VirtualBox (VirtualBox 6.0). I tried installing the aforementioned packages but had no luck. My solution was simpler than I had anticipated. While the machine is powered off, go to:

Settings (of the VM) > Display > Graphics Controller > and select "VBoxVGA"

VirtualBox had set this option to "VMSVGA" by default for me. Fortunately, I noticed this subtle difference when comparing the settings between some of my virtual machines. I hope this works for you as well!

just type: sudo apt-get install virtualbox-guest-additions-iso

Install the latest version of virtualbox say 4.3.10 or above from here and also it's corresponding guest additions, extensions Or you can also install the latest virtualbox version from the multiverse repository via apt-get. I also faced the same problem in older virtualbox version running Ubuntu 14.04 as guest OS.

To install guest additions, follow this answer.