When you install macOS in Virtual Box, the screen resolution is predefined, say It’s 1024×768. You are not able to change it via Virtual Box settings.
In this article, I will tell you how to change the Screen Resolution of macOS in Virtual Box within two steps.
We are not able to change screen resolution via Virtual Box user interface
How to change screen resolution
Step 1: Open command prompt, run as Administrator
Step 2: Run flowing commands to change screen resolution to 1440×900
cd "C:\Program Files\Oracle\VirtualBox\"
VBoxManage setextradata "Your Virtual Machine Name" VBoxInternal2/EfiGraphicsResolution 1440x900
Step 3: Start your machine and enjoy the result
Hung Doan 
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