How should I increase the spinner list item font size?

Via XML Only

Just to help others in case they are statically setting their Spinner entries in XML.

The above answers work if you're creating your Spinner via code but if you're setting your Spinner entries via XML, i.e. using android:entries, then you can adjust the text size and other attributes with the following two theme settings:

In your res/values/styles.xml

<?xml version="1.0" encoding="utf-8"?>
<resources>

    <style name="AppBaseTheme" parent="android:Theme.Holo">
    </style>

    <!-- Application theme. -->
    <style name="AppTheme" parent="AppBaseTheme">

        <!-- For the resting Spinner style -->
        <item name="android:spinnerItemStyle">
            @style/spinnerItemStyle
        </item> 

        <!-- For each individual Spinner list item once clicked on -->
        <item name="android:spinnerDropDownItemStyle">
            @style/spinnerDropDownItemStyle
        </item>

    </style>

    <style name="spinnerItemStyle">
        <item name="android:padding">10dp</item>
        <item name="android:textSize">20sp</item>
        <item name="android:textColor">#FFFFFF</item>
    </style>

    <style name="spinnerDropDownItemStyle">
        <item name="android:padding">20dp</item>
        <item name="android:textSize">30sp</item>
        <item name="android:textColor">#FFFFFF</item>
    </style>

</resources>

The above solutions are all hard coded in the xml.

There is an alternative solution which allows you to change it programmatically. https://stackoverflow.com/a/11494962/5089713 Once you have the TextView, you can do whatever with it, for instance change the font size.

your code then looks like

Spinner food = (Spinner) findViewById(R.id.spinner1);

CharSequence[] strings = getActivity().getResources().getTextArray(R.array.item_array);
    ArrayAdapter<CharSequence> foodadapter = new ArrayAdapter<CharSequence>(getActivity(), android.R.layout.simple_spinner_item, strings){
        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            View view =super.getView(position, convertView, parent);
            TextView textView=(TextView) view.findViewById(android.R.id.text1);
            // do whatever you want with this text view
            textView.setTextSize(20);
            return view;
        }
    };

foodadapter.setDropDownViewResource(android.R.layout.simple_spinner_item);        
food.setAdapter(foodadapter);

Save the below xml as spinner_layout.xml in layout folder

<?xml version="1.0" encoding="utf-8"?>
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
          android:id="@+id/spinnerTarget"
          android:layout_width="fill_parent"
          android:layout_height="wrap_content"
          android:textColor="#000000"
          android:textSize="13sp"
/>

change the textSize which you want. and use the below adapter code to fill it.

Spinner food = (Spinner) findViewById(R.id.spinner1);
ArrayAdapter<CharSequence> foodadapter = ArrayAdapter.createFromResource(
            this, R.array.item_array, R.layout.spinner_layout);
foodadapter.setDropDownViewResource(R.layout.spinner_layout);
food.setAdapter(foodadapter);