How do I change the text style of a spinner?

Via XML Only

As a follow-up to @Cyril REAL's excellent answer, here is a thorough implementation of how to style your Spinners just through XML if you're populating your Spinner via android:entries.

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>

As my predecessor specified, you can't do it on the main XML layout file where the Spinner component is.

And the answer above is nice, but if we want to use Google's best practices, like you know... to use styles for everything... you could do it in 3 'easy' steps as follows:

Step 1: You need an extra file under your layout folder with the look for the Spinner's items:

<?xml version="1.0" encoding="utf-8"?>
<TextView  
    android:id="@+id/textViewSpinnerItem" 
    android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    style="@style/SpinnerTextViewItem"
    xmlns:android="http://schemas.android.com/apk/res/android" />

Name this file: spinner_item_text.xml

Step 2: Then, on your Activity Class when you are filling the Spinner with an array of items:

adapter = new ArrayAdapter<CharSequence>(this, R.layout.spinner_item_text, items);
spinner.setAdapter(adapter);

Note that the R.layout.spinner_item_text resource is in your own R's file.

Step 3: Under your values folder, create or use (you might have one already) the file styles.xml. The style entry needed should look like this one:

<style name="SpinnerTextViewItem" parent="@android:style/Widget.TextView" >
    <item name="android:textSize" >8dp</item>
    <item name="android:textStyle" >bold</item>
</style>

And that's it!

So far it has been really, really handy to put all about text sizes, styles, colors, etc... on a styles.xml file so it's easy to maintain.