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: () {}),
        ),
      ),

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

final 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: () {},
  ),
);

You do not have to reinvent the wheel, flutter team knew it will be needed.

Instead of using regular FloatingActionButton() use FloatingActionButton.extended()

example code:

FloatingActionButton.extended(
  onPressed: () {},
  icon: Icon(Icons.save),
  label: Text("DOWNLOAD"),
),

src: proandroiddev