Height and Width
A component's height and width determine its size on the screen.
Fixed Dimensions
The general way to set the dimensions of a component is by adding a fixed width and height to style. All dimensions in React Native are unitless, and represent density-independent pixels.
Setting dimensions this way is common for components whose size should always be fixed to a number of points and not calculated based on screen size.
There is no universal mapping from points to physical units of measurement. This means that a component with fixed dimensions might not have the same physical size, across different devices and screen sizes. However, this difference is unnoticeable for most use cases.
Flex Dimensions
Use flex in a component's style to have the component expand and shrink dynamically based on available space. Normally you will use flex: 1, which tells a component to fill all available space, shared evenly amongst other components with the same parent. The larger the flex given, the higher the ratio of space a component will take compared to its siblings.
A component can only expand to fill available space if its parent has dimensions greater than 0. If a parent does not have either a fixed width and height or flex, the parent will have dimensions of 0 and the flex children will not be visible.
After you can control a component's size, the next step is to learn how to lay it out on the screen.
Percentage Dimensions
If you want to fill a certain portion of the screen, but you don't want to use the flex layout, you can use percentage values in the component's style. Similar to flex dimensions, percentage dimensions require parent with a defined size.