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.