问题:

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

<menu xmlns:android="http://schemas.android.com/apk/res/android" >
    <item
        android:id="@+id/m_login"
        android:enabled="true"
        android:showAsAction="always"
        android:icon="@drawable/login"/>
</menu>

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

Here's the icon:

回答1:

First of all I suggest you to try with different sizes of the icon which you want, without no paddings until you achieve your goal. If that not suits you, you can add a custom view in actionbar and set OnClickListener on a button and achieve your goal. Here is a way how to do this :

View customNav = LayoutInflater.from(this).inflate(R.layout.my_button, null); // layout which contains your button.
Button mButton = (Button) customNav.findViewById(R.id.date_num);
mButton.setOnClickListener(this);

getSupportActionBar().setCustomView(customNav);
getSupportActionBar().setDisplayShowCustomEnabled(true);

That should do the trick. You can use Button or ImageButton to achieve this.

回答2:

Use high resolution transparent PNG images with no padding. Follow the Icon Design Guidelines. You can also use the Action Bar and Tab Icon Generator to generate icons specific to the guidelines.