proper way to get widget height in SafeArea

You can always use LayoutBuilder for such cases.

child: SafeArea(
        child: new LayoutBuilder(
            builder: (BuildContext context, BoxConstraints constraints) {
              // constraints variable has the size info
              return Container();
            }
        ),
      ),

for more info: https://www.youtube.com/watch?v=IYDVcriKjsw

I know that there is right answer, but maybe someone is looking for safe area height, not height of safe area widget child, but only safe area top padding:

var safePadding = MediaQuery.of(context).padding.top;