This is often a security restriction within Windows.
Installing Guest Additions improves the VM experience by providing: Chapter 4. Guest Additions - Oracle VirtualBox
If you installed Guest Additions but the screen is still small: download virtualbox guest additions
The safest and most compatible source is your existing VirtualBox installation.
Select from the dropdown. Method 2: Manual Download (Fallback) This is often a security restriction within Windows
Most users do not need to manually download a file from the internet. VirtualBox includes the Guest Additions ISO image within its installation directory.
A virtual CD is already locked inside the virtual optical drive. Select from the dropdown
The official "paper" or documentation detailing this process is , which you can access through the VirtualBox Manual (Chapter 4) Key Methods to "Download" or Mount Guest Additions
sudo apt install build-essential dkms linux-headers-$(uname -r) Use code with caution.