Check this link:
ActionBarSherlock - How to set the padding of each actionbar's icon?
The idea is to create a new theme which has a parent theme as Theme.Sherlock
and lower actionButtonStyle minWidth
attribute
<style name="MyTheme" parent="Theme.Sherlock">
<item name="actionButtonStyle">@style/MyActionButtonStyle</item>
</style>
<style name="MyActionButtonStyle" parent="Widget.Sherlock.ActionButton">
<item name="android:minWidth">28dip</item>
</style>
and apply this theme to your Activity in the manifest.