We apologize for having trouble with image size. In the theme, we use image size of 400X225 (16:9) in the blog posts section and 1056X594(16:9) on a single page.
You can use any higher resolution image with a 16:9 ratio. WordPress will automatically create both thumbnails.
Note: If you are using existing uploaded images then you need to regenerate your thumbnail to create the above 2 images size. To regenerate thumbnails you can use plugins like https://wordpress.org/plugins/regenerate-thumbnails/
Hope this helps.
Please feel free to reach back to us if you have any issues further.