I have also seen other tutorials instructing to use the above without putting VBoxInternal2/EfiGraphicsResolution in quotes.

You should edit 1920x1080 to the resolution you need, and the name of your vm. Then you can use the code VBoxManage setextradata 'Whatever Your Machine Is Called– vmname, put in these quotes' 'VBoxInternal2/EfiGraphicsResolution' '1920x1080' cd to where your VirtualBox folder is located, I believe the default is in the C drive, but mine is put in E intentionally so you would do that by directing to the folder as such: Run the Command Prompt as Administrator. So maybe download them if the commands to follow do not work. I do not know if VMwareTools (press X to doubt) and Virtual Box Darwin Guest Additions (this was actually specified for Mac OS X so I'm not sure) are necessary, but what I tried did not work before I downloaded them. Refer to the following Similar Forum, which did not work for me, first. Maybe someone can also explain why what I did works. I hope that others who found different solutions, especially when the solutions I tried did not work, can share theirs. This question is for people who relate to the same problems I have and spent hours searching the internet to no avail. Disclaimer- I just got this working for me in my Virtual Box 6 Big Sur guest OS version 11.1.

Virtualbox full screen mac os x