You can edit those posts, and in the right corner below at the last position, you will find the option of Featured Image. There, can you upload the image size as of the above mentioned and check?
But, if you do not want to edit the posts and change featured image, and can code it on your own, then, you can create a child theme and then dequeue the parent theme script, which handle name is
the-newsmag-main-script. Now, you need to again enqueue this same file via child theme, with any other handle name, which resides in js folder. Now, here, copy the file js/the-newsmag-custom.js to your child theme and then find the code:
adaptiveHeight: true,, which need to be changed to
adaptiveHeight: false, and check.
If you like the theme and appreciate the work put out on the theme, then, you can help by rating this theme through this link here: https://wordpress.org/support/theme/the-newsmag/reviews/