WordPress has a couple of "default" sizes (i.e. not defined in a theme's
functions.php), which are automatically generated: "Thumbnail" (150px) and "Medium" (300px). You can see this when you upload or edit an image.
It would be really good if the Crop Thumbnails plugin could detect and edit these default sizes as well. I don't know how easy this would be, but there was an old plugin called Post Thumbnail Editor that could do this.