Select your interactive element on one of the 8 points around it. Your mouse will change in a double arrow button. Then extend or reduce the size of your interactive element.
It is also possible to change the size of a hotspot or a popup in the geometry tab. You can directly enter the width and the height of your interactive element. By default, the width and the height are relative to the size of the video, but you can also make them relative to the size of the player, or you can make it fixed size. If you select the fixed size option, it will be a size in pixels.