flutter, how to align the cross ICON to center position?

SizedBox(
        width: 20,
        height: 20,
        child: FloatingActionButton(
          onPressed: null,
          child: Icon(Icons.add),
        ),
),

I want to size Button and also want to put the ICON to center position, but it aligns bottomright little bit… how can I do it? (button size should be 20×20)

my code result picture

>Solution :

Try to use Container instead of SizedBox and set Alignment.center, refer Align:

   Container(
            alignment: Alignment.center,
            width: 20,
            height: 20,
            child: FloatingActionButton(
              onPressed: null,
              child: Icon(
                Icons.add,
                size: 15,
              ),
            ),
          ),

Result-

Other way:

     FloatingActionButton(
            onPressed: null,
            child: Icon(Icons.add),
          ),

Result->