What is the size of ActionBar in pixels?

To retrieve the height of the ActionBar in XML, just use

?android:attr/actionBarSize

or if you're an ActionBarSherlock or AppCompat user, use this

?attr/actionBarSize

If you need this value at runtime, use this

final TypedArray styledAttributes = getContext().getTheme().obtainStyledAttributes(
                    new int[] { android.R.attr.actionBarSize });
mActionBarSize = (int) styledAttributes.getDimension(0, 0);
styledAttributes.recycle();

If you need to understand where this is defined:

1. The attribute name itself is defined in the platform's /res/values/attrs.xml
2. The platform's themes.xml picks this attribute and assigns a value to it.
3. The value assigned in step 2 depends on different device sizes, which are defined in various dimens.xml files in the platform, ie. core/res/res/values-sw600dp/dimens.xml

One of the Honeycomb samples refers to ?android:attr/actionBarSize

To get the actual height of the Actionbar, you have to resolve the attribute actionBarSize at runtime.

TypedValue tv = new TypedValue();
context.getTheme().resolveAttribute(android.R.attr.actionBarSize, tv, true);
int actionBarHeight = getResources().getDimensionPixelSize(tv.resourceId);

From the de-compiled sources of Android 3.2's framework-res.apk, res/values/styles.xml contains:

<style name="Theme.Holo">
    <!-- ... -->
    <item name="actionBarSize">56.0dip</item>
    <!-- ... -->
</style>

3.0 and 3.1 seem to be the same (at least from AOSP)...