Hello simt,
Thank you so much for using our theme. In case of a featured image, did you check the options for featured image size on customizer?
1. Go to Appearance => Customize => Layout/Design Option => Default Blog/Archive Layout => Image Layout( Size ) Options and choose Image size to display
2. For single post Go to Appearance => Customize => Single Post Options => Image Layout ( Size ) Options and choose Image size to display.
Maybe that is helpful options for you. Kindly check the settings on customizer for single post options and try changing the image size.
Thank you