Featured image sizes are often the topic of conversation in a variety of forums. First, the featured image size is usually set in your theme. Typically, the theme creates at least three images from a single image you upload, thumbnail, medium and large. Many complex themes create many more than three images from each image that you upload and use that same image in a variety of places.
Also, in the theme templates themselves, this is where the image is being placed along with the decision to what size is being used. So, for example, you may upload a single image and three images are created, one might be used in the header of the article, one might be used in a blog listing page and use only the thumbnail.
What I typically do, is figure out where the full sized image is being used and create my images in PhotoShop so that the largest version of the image is the cleanest and clearest, and let WordPress do the rest of the work for you.
I am not sure if this answers your question exactly, but the bottom line is that that theme itself usually dictates the different image sizes being used. Even if you go into your admin area and create specific image sizes for thumbnail, medium and large, the theme templates control what sizes are actually being used – unless you modify this code yourself.
Good luck with your project.
Bob
