How to change the size of FloatingActionButton?

wrap your FAB with a FittedBox inside a Container or SizedBox and then change the width and the height of it.

example :

floatingActionButton: Container(
        height: 100.0,
        width: 100.0,
        child: FittedBox(
          child: FloatingActionButton(onPressed: () {}),
        ),
      ),