Kali display too small in VirtualBox
Solution 1:
You need to add VBOX guest extensions to the Kali machine. That should allow a range of resolutions to fit the machine to your screen.
I am posting this from a Kali 2021.2 machine with a resolution of 1265x800 (machine fits VMware window).
I am using VMware but the concept (Guest Extensions or VMware Tools) is about the same.
Guest extensions
kali@kali:~$ sudo apt update ...
kali@kali:~$ sudo apt install -y --reinstall virtualbox-guest-x11 ...
kali@kali:~$ sudo reboot -f
I am running Kali in VirtualBox (using a pre-made Kali VDI found on their website)
Followup Alternative Approach:
You may be wiser to download the Kali ISO (Full ISO). Then make your Kali machine from that. The installation may install the Guest Extensions properly.
I did this and I recommend it as better approach.