I had the same problem.
Apparently it only has something to do with your CSS!
I cross checked it with the setting on the widget. For example, every time I changed the size to 200px nothing happend visually. BUT then I looked in HTML source code.
<img width="150" height="200" src="http://..." class="attachment-200x200 wp-post-image" alt="jacket">
So the HTML code is changing! Hurray.
Then I took a look at the row thing and changed it from 3 to 4 items per row.
Again. Nothing happend. Visually.
But you can check the change in the HTML code.
If you want to have 4 items, the code needs to like like this:
<div class="feature first">...</div>
<div class="feature last">...</div>
As you can see, the first and the last div-item got an extra class. So this is working. Hurray again.
And now you need to play around in CSS with the width of the "feature" classes. And maybe with the img-tags inside these classes.