How to set size to 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: () {}),
        ),
      ),

Use a Container where you can specify width and height, then use a RawMaterialButton, like this:

myFabButton = Container(
      width: 200.0,
      height: 200.0,
      child: new RawMaterialButton(
        shape: new CircleBorder(),
        elevation: 0.0,
        child: Icon(
          Icons.favorite,
          color: Colors.blue,
         ),
      onPressed: (){},
  );

Use a SizedBox

SizedBox(
  width: 200.0,
  height: 200.0,
  child: FloatingActionButton(
    onPressed: () {},
  ),
)