Width of text column vs overhanging image (with sidebar)

I’m a little confused about the various widths of things in this theme.

When you’ve got a sidebar, featured images are the same width as overhanging images (840px), right? Whereas, without a sidebar, they’re wider (1200px)?

I’m confused because the recommended width (on the WordPress.com page) for a featured image is 1200px. But maybe that’s based on no sidebar?

To be clear, I have two questions:

* What is the width of a featured image *when you have a sidebar*?
* What is the width of the text column *when you have a sidebar*?

I’m just trying to figure out what width to make my featured images versus those that I *don’t* want to overhang in the body of the text. The WordPress.com page seems suggests that if you make images in the body-text area 839px or narrower, they won’t overhang, but is that the actual width of that area?

I hope that makes sense!