Android button font size

Button butt= new Button(_context);
butt.setTextAppearance(_context, R.style.ButtonFontStyle);

and in res/values/style.xml

<resources>   
    <style name="ButtonFontStyle">
            <item name="android:textSize">12sp</item>
    </style>
</resources>

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.