1. Click the Run button.

2. Wrap the second BlueBox widget in an Expanded widget.

For example:

Expanded

child:

BlueBox

(),),

3. Select the Format button to properly format the code,
         and run again.

SizedBox widget

Example: Resizing a widget

The following example wraps the middle BlueBox widget inside of a
  SizedBox widget and sets the BlueBox’s width to 100 logical pixels.

1. Click the Run button.

2. Add a height property equal to 100 logical pixels
         inside the SizedBox widget, and run again.

Example: Creating space