Here we recommend to take the approach of making it as large as the largest display.
If the image is displayed on desktop in a 2-column layout, then about 700px width should be enough. In the smaller devices it would be displayed smaller and then "weighs" not too heavy in terms of traffic.
Note: A snapshot from the cell phone camera can have a resolution of 20 MP and then be 5 MiB and larger -> Nobody wants to load that over the mobile network
Tips for content
Regarding the content, keep in mind that most mobile screens are thin and long. To make it scannable, use headings, bullet points and avoid white space to organize your content.If possible, keep your paragraphs nice and tight.