I'm recording educational screencasts with Virtualbox. Because Virtualbox guest OS window resizes itself with the screen resolution of guest OS, I can't set a constant recording area with my screen recording software. When I set a recording size for, let's say, Virtualbox boot up screen window size, it gets too small a few moments later. If I go the opposite way and set the window size for a full guest OS window, it gets too large when I reboot the guest -and my host computer's desktop is seen clearly on the screencast.
What I need is being able to set a constant window size. It could be treated as an LCD screen like, whatever resolution is inside, it has the same size and if guest OS resolution is smaller than that of the Virtualbox VM window size (which is preset in pixels), guest OS screen is shown with all black elsewhere in VM window.
Going fullscreen with Ctrl+F could work but then I have performance issues because resolution gets too large for my system to handle the recording process while using the VM.
Thanks in advance.