Disclaimer-- I just got this working for me in my Virtual Box 6 Big Sur guest OS version 11.1... This question is for people who relate to the same problems I have and spent hours searching the internet to no avail. I hope that others who found different solutions, especially when the solutions I tried did not work, can share theirs. Maybe someone can also explain why what I did works.
Refer to the following Similar Forum, which did not work for me, first.
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. So maybe download them if the commands to follow do not work.
These are the steps I did: <I have a Windows 10 host computer>
-- Power off your virtual mac OS.
-- Run the Command Prompt as Administrator.
-- 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:
cd Program Files/Oracle/VirtualBox
-- Then you can use the code
VBoxManage setextradata "Whatever Your Machine Is Called– vmname, put in these quotes" "VBoxInternal2/EfiGraphicsResolution" "1920x1080"
You should edit 1920x1080 to the resolution you need, and the name of your vm. I have also seen other tutorials instructing to use the above without putting VBoxInternal2/EfiGraphicsResolution in quotes. Didn't work for me. After putting the code, open up your vm and hopefully reso is changed.
NOW, I've painfully tried to make this work with 1366x768, but it did NOT work (it goes to the default reso instead). It only worked for certain standard resolutions where 1366x768 is not a part of. I found this after a while, and it is probably why it didn't work. It wasn't supported as a standard resolution. So, I found this and created a custom mode through the following:
VBoxManage setextradata "vmname again" "CustomVideoMode1" "1366x768x32"
After doing so, I tried again, to put the first block of code:
VBoxManage setextradata "Whatever Your Machine Is Called– vmname, put in these quotes" "VBoxInternal2/EfiGraphicsResolution" "1366x768"
It worked. I am assuming it has to do with creating your own standard resolution, and only then can you use that resolution? Anyone confirming would be nice. Forgive me if I'm completely off the mark, this is my first time using virtual box properly.
I just realized there is an answer my question option at the end.