Yes your question makes sense. My understanding of the image attachments in WP is that you upload an image and it gets resized to all defined sizes currently defined in your instance of WP.
So you can’t specify a given size for a given image upload. And therefore, there is no way of ascertaining what a specific image’s size should be.
I expect the size you use is determined by its location in your theme, and specified by you in the theme. (Assuming, of course, you’re the themer.)