As Kelemelan pointed out, the cogwheel (but I like the sun too) icon in the upper right corner is there to toggle between different window sizes. You can also press alt+enter to toggle between Fullscreen and Borderless window easily. If you move around the software in a windowed position and then switch it back to fullscreen, it should switch to fullscreen on the screen it is currently present – but this is up to the operating system to either respect or discard this request.
I hope this helps, but please let us know if there’s any other issues with this.