Have you tried with different ways of arranging automatically the windows?
https://neuroimage.usc.edu/brainstorm/Tutorials/MultipleWindows
Have you tried setting the interface scaling factor to 100%? (from the Brainstorm preferences)
https://neuroimage.usc.edu/brainstorm/Tutorials/CreateProtocol#The_text_is_too_small
If you have multiple screens: Try working on one screen only.
Can you please post a screen capture showing what you desktop looks like at the moment you start making this video (make sure we see the popup menu you click to make the video)