If you click on an image inside the StoryChief editor, you are able to change the display size. The following sizes are available:
Full-width (only available if the image is over 2000px wide)
In your WordPress theme, you are able to customize the exact values that will be used for your published stories. To do this, complete the following steps:
Start customizing your image sizes
Step 1. Login to the admin side of your WordPress website.
Step 2. Via the sidebar, go to "Appearance" ➜ "Customize".
Step 3. Click on "Additional CSS".
Step 4. Add the following CSS styling and click on "Publish". Feel free to customize the values to your preferences.