I need to dig into the Dawn theme code a bit. But I imagine you would do this with a little code editing and metafeilds. I do not see a way to do this without editing code. There is an option to change the size/ratio of the internal image through the settings. This might be a good temporary solution.
If you are interested in looking into those settings you will find them by following these steps:
Online Store (click)
Customize Button (click)
(click) Search Bar and type in blog into the editor screen hit enter (this will open up your blog)
(click) into a blog article
(click) the banner image and try out the difference sizes shown in the menu.
This can also be customized more with CSS changes which is a bit easier than changing the logic of the page.