How can I increase the size of the VirtualBox screen?

Your screen indicates the need to install Virtual Box Guest Extensions in the Ubuntu Machine.

This should augment video and allow the Ubuntu screen to fill the VBOX screen.

Here is a nice article to help you.

Prerequisites Install the build tools and kernel headers for the
current kernel.

COPY sudo apt update

sudo apt install -y build-essential linux-headers-$(uname -r) Install
Guest Additions After attaching VirtualBox guest additions ISO image,
install it either using a terminal or graphical mode.

Using Terminal On the VM window, go to Devices » Insert Guest
Additions CD Image.

Open a terminal Ctrl+Alt+T and mount the ISO image to /media

COPY sudo mount /dev/cdrom /media Execute the VirtualBox guest
addition installer.

COPY cd /media

sudo ./ Even before the installation gets
complete, your system will get the full-screen view which confirms
that the installation is successful.

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