Adjust icon size of Floating action button (fab)

The new floating action button should be 56dp x 56dp and the icon inside it should be 24dp x 24dp. So the space between icon and button should be 16dp.

<ImageButton
    android:id="@+id/fab_add"
    android:layout_width="56dp"
    android:layout_height="56dp"
    android:layout_gravity="bottom|right"
    android:layout_marginBottom="16dp"
    android:layout_marginRight="16dp"
    android:background="@drawable/ripple_oval"
    android:elevation="8dp"
    android:src="https://stackoverflow.com/questions/27484126/@drawable/ic_add_black_48dp" />

ripple_oval.xml

<ripple xmlns:android="http://schemas.android.com/apk/res/android"
    android:color="?android:colorControlHighlight">
    <item>
        <shape android:shape="oval">
            <solid android:color="?android:colorAccent" />
        </shape>
    </item>
</ripple>

And this is the result I get:

I used the icon from \material-design-icons-1.0.0\content\drawable-hdpi\ic_add_black_48dp.png
https://github.com/google/material-design-icons/releases/tag/1.0.1

How to make the size of the icon inside the button be exactly as described in guidelines ?

http://www.google.com/design/spec/components/buttons.html#buttons-floating-action-button