Let me tell you that the available space can be measure or calculate by using Layout builder and Media Query. These can help to know the screen size of your device.

Required Space, which means the widget that develop and occupied some screen space in your device screen. This can be decided by developers like 20 pixel height or width, 20% of screen space. This is how you can decide the required space for the widget in advance. It completely choice of developer based on available space or device screen.