Widget: “Hey second child, You must be from 0 to 290 wide,
   and 0 to 55 tall.”

Second child: “OK, I wish to be 140 pixels wide,
   and 30 pixels tall.”

Widget: “Very well. My first child has position x: 5 and y: 5,
   and my second child has x: 80 and y: 25.”

Widget: “Hey parent, I’ve decided that my size is going to be 300
   pixels wide, and 60 pixels tall.”

Limitations

As a result of the layout rule mentioned above,
Flutter’s layout engine has a few important limitations:

A widget can decide its own size only within the
constraints given to it by its parent.
This means a widget usually can’t have any
size it wants.