You define these attributes in xml as you would anything else, for example:
<Button android:id="@+id/next_button"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/next"
android:background="@drawable/mybutton_background"
android:textSize="10sp" /> <!-- Use SP(Scale Independent Pixel) -->
You can find the allowed attributes in the api.
Or, if you want this to apply to all buttons in your application, create a style. See the Styles and Themes development documentation.