you can do as they say in the comments or you can save the effort and work with RawMaterialButton . which have everything and you can change the border to be circular and alot of other attributes. ex shape(increase the radius to have more circular shape)
shape: new RoundedRectangleBorder(borderRadius: BorderRadius.circular(25)),//ex add 1000 instead of 25
and you can use whatever shape you want as a button by using GestureDetector which is a widget and accepts another widget under child attribute. like in the other example here
GestureDetector(
onTap: () {//handle the press action here }
child:Container(
height: 80,
width: 80,
child:new Card(
color: Colors.blue,
shape: new RoundedRectangleBorder(borderRadius: BorderRadius.circular(25)),
elevation: 0.0,
child: Icon(Icons.add,color: Colors.white,),
),
)
)