[android help] How to increase icon size of actionButton in SherlockActionBar?

To define the actionButton and set its icon, I would:

            android:id="@+id/m_login"
        android:enabled="true"
        android:showAsAction="always"
        android:icon="@drawable/login"/>

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The resulted icon(drawable/ldpi) looks very small by default: (Any way to increase the size?)

Here's the icon:

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