I had just installed VirtualBox 6.0 on MacOS (MacbookPro 2018), created a VM and installed Ubuntu 18.10 as the Guest OS. When I started my VM, the ubuntu was a small 400X400 window in the center of the large 2880×1880 Mac resolution.
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I had to do the following to fix my resolution.
1. Click on Tools
If you don’t see Tools, you may need to download the extension pack from VirtualBox Downloads Page
2. Click on Preferences
3. Click on Display
4. Ensure None is selected for Maximum Guest Screen Size
Next we need to change the Virtual Screen value on the VM itself.
5. Double Click on the VM
6. Go to Menu->View->Virtual Screen1 Select the autoscaled 200% setting
The above worked for me. Please leave a comment if it worked for you too.