preferredSize property
override
The size this widget would prefer if it were otherwise unconstrained.
In many cases it's only necessary to define one preferred dimension.
For example the Scaffold only depends on its app bar's preferred
height. In that case implementations of this method can just return
new Size.fromHeight(myAppBarHeight)
;
Implementation
@override
Size get preferredSize => const Size.fromHeight(_kTabBarHeight);