Flutter rounded profile image appBar. Consider using Material combined with a shape: new CircleBorder() instead of manually creating a circle. Or a CircleAvatar if that fits your needs.
Then add a Padding to control the size and margin.
return new Scaffold(
appBar: new AppBar(
title: new Text("Activities"),
leading: new Padding(
padding: const EdgeInsets.all(8.0),
child: new Material(
shape: new CircleBorder(),
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