I installed Ubuntu 14.04 LTS amd64 desktop on VirtualBox 4.3.26 which is running on OSX10.10 (amd64) and the only option for screen resolution is 640x480. This resolution is found under the System Settings -> Displays -> Resolution drop down menu.
I've tried installing the dkms package and virtualbox-guest-additions (which wasn't able to be found, maybe my sources.list file is bad?) as suggested in other forums, but it didn't help.
This is all freshly installed, I haven't tampered with the machine or settings at all.
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.
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
Here I added the numbers from my display resolution so now the linux in VB became as large as the screen. But with very poor resolution.
Then in VB - in linux - I had to set the resolution again to same numbers (it was 1920 x 1080 for me)
Had this problem running Ubuntu 20.04 in virtualbox. This was my first time running a vm. Everytime I went full-screen the resolution was just awful. I did everything everyone above said, but what finally fixed it was I had to go into the actual display settings in Ubuntu. I changed them to 1920x1080 for my hd display. That was what worked.
Going into Seamless mode helped me. You can Right-Click and press L or go to the top header and click Seamless Mode. I had also previously followed user s g's steps, but nothing worked till I entered Seamless Mode.
If you still experience problems with setting resolution after setting resolution resizing to 'None' or 'Automatic' in VirtualBox -> Preferences -> Display and mounting GuestAdditions execute following command in guest's terminal: