By default, FL Studio is maximized to cover the whole screen. When you click the max/minimize button to make it smaller, it will stay the same size. There's no obvious way to make it smaller
Follow these steps:
1. If the maximize/restore button looks like image A, click on it. It will then look like image B.
2. Move the window to a different position by dragging the title bar
3. Once it's moved, resize it by dragging a border
Image A
Image B
...and a video:
The FL Studio Support Team.