thanks for the compliments. It's alway good to know, people like what you do. But I think I can't reproduce your problem. Or I missunderstand.
What the plugin does, is taking the featured image of a post, if there is no width given. As soon as you change the size of the image shown, will be counted upon display and the middle-sized image of the attached image of that post is taken and sized down.
But it should be sized down. So if your thumbnails are 150px wide, there is actually no need to give a size in the widget.
Do you have a URL for me? That I could check out, whether or not the plugin is behaving as it should.