How can I change the style of a ProgressBar to small?

The solution is to change the style to

<ProgressBar
   android:layout_width="wrap_content"
   android:layout_height="wrap_content"
   style="?android:attr/progressBarStyleSmall" />

One more way to do it is

style="@android:style/Widget.ProgressBar.Small" 

Ex_

<ProgressBar
 android:id="@+id/progress_bar"
 style="@android:style/Widget.ProgressBar.Small"
 android:layout_width="wrap_content"
 android:layout_height="wrap_content"
 android:padding="7dip"
 arandroid:visibility="visible" />

Here is how to do it programmatically:

ProgressBar progress = new ProgressBar(ctx, null, android.R.attr.progressBarStyleSmall);

This may be of use:

style="?android:attr/android:progressBarStyleSmall"