I'm trying to have 2 monitors showing 2 different things (one is streaming a video full size and the other is my Photoshop). But everytime I start to work on my PS, the video shrinks back to normal size. How do I fix this?
I used to have two monitors and I had the same issue as you, but there are some multiple monitor management tools (I think their mainly commercial products) that may be able to help resolve that kind of issue from occurring. I currently (for example) use a product called Actual Window Manager which can help control multiple screens and it may be able to help you get the effect you require (though I would try the trial before buying to make sure). Why it's occurring is because clicking outside the display region (where the full screen effect is not apply) causes a window to restore back to a non-full state, and as the second monitor is effectively an expansion of the desktop clicking causes the restoration. It may come to forcing the full screen mode to stick using keyboard shortcuts or something similar.