How to set an equal image size for a Fallback Image as used for the Featured Image(dynamicly)

You can get the width and height of the image size set back from the global $_wp_additional_image_sizes array via the image size slug, so that:

global $_wp_additional_image_sizes;
$size="featured-image";
$width  = $_wp_additional_image_sizes[$size]['width'];
$height = $_wp_additional_image_sizes[$size]['height'];

Then in your template just change the image attributes:

<img src="https://wordpress.stackexchange.com/questions/217134/<?php bloginfo("template_directory'); >/images/thumb_featured_img.jpg" width="<?php echo $width; ?>" height="<?php echo $height; ?>" alt="<?php the_title(); ?>" />