Iris window mode doesn't allow you to shrink/expand the window unfortunately. It uses fixed sizes, and if your monitor isn't massive, none of them are really small enough to allow you to have your browser or other app side by side.

One thing you could try though. See if your video playing application has an "Always on top" setting so that it stays on top of the Iris window and then you can pull it up to the top left. It might hide your minimap but it's better than nothing right