after some tests, there is an easier way than subclassing ArrayAdapter. Change the
ArrayAdapter<CharSequence> adapter = ArrayAdapter.createFromResource(this, R.array.planets_array,android.R.layout.simple_spinner_item);//this is from the tutorial, adapt with your line
to this line :
ArrayAdapter<CharSequence> adapter = ArrayAdapter.createFromResource(this, R.array.planets_array, R.layout.textview);
You just have to change the design of the TextView you use of the Spinner. I just tried with this, as the layout of the textview.xml :
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
android:text="@+id/TextView01" android:id="@+id/TextView01"
android:layout_width="wrap_content" android:layout_height="wrap_content" android:textSize="30sp"></TextView>