Hi @Emory,
Clean Journal Pro theme has multiple layouts, and size for main column varies with respect to the layout option. In your context, you have main content, and a primary sidebar. For this layout, the default widths are as below:
main : 820px
sidebar : 340px
wrapper : 1200px
Default width of wrapper is 1200px which remains the same for all layouts.
As you’ve mentioned, its not a good idea to directly change the code if you want to change some styles, you can always use the Custom CSS in “Appearance => Theme Options => Custom CSS” box or create child theme and add in your child theme style.css
file
For featured image, its not that critically important to stick with the image size 780 x 586, you can always use the images larger than this. But you need to make sure the image’s aspect ration is 4:3 because the image will be automatically cropped when uploading. So if the aspect ration mismatches, it may crop some portion of the image itself. If you use the small sized image then it will use the default uploaded image.
If you need any further help on CSS for the above, please let me know.
Thank you for your appreciation.