Tip:
  MainAxisSize.max is the mainAxisSize property’s default value.
  If you don’t specify another value,
  the default value is used,
  as shown in the previous example.

Example: Modifying axis size

The following example explicitly sets mainAxisSize
  to its default value, MainAxisSize.max.

1. Click the Run button.

2. Change MainAxisSize.max to MainAxisSize.min,
         and run again.

mainAxisAlignment property

Positions children near the beginning of the main axis.
(Left for Row, top for Column)

Positions children near the end of the main axis.
(Right for Row, bottom for Column)

Positions children at the middle of the main axis.

Divides the extra space evenly between children.

Divides the extra space evenly between children
and before and after the children.