Use app:fabCustomSize.

<com.google.android.material.floatingactionbutton.FloatingActionButton
    android:id="@+id/floatingActionButton"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    app:srcCompat="@drawable/ic_launcher_foreground"
    app:fabCustomSize="74dp" />

https://github.com/material-components/material-components-android/blob/master/docs/components/FloatingActionButton.md#sizing-fabs