Changing Size of a set Image

The best way to update the add_image_size is to do it in a child theme functions file so you don't edit any core files.

I found an article on that here: http://wordpress.stackexchange.com/questions/74934/remove-or-update-add-image-size

So something perhaps like this on your child theme functions file.

function child_theme_setup() {
add_image_size( 'blog_image', 668, 180, true ); //(cropped) 
}
add_action( 'after_setup_theme', 'child_theme_setup', 11 );

Here are all the image sizes used:

themo_full_width
themo_full_width
themo_thumb_slider
themo_thumb_slider_portrait
themo_brands
themo_mini_brands
themo_testimonials
themo_featured
themo_team
themo_showcase
themo_page_header
themo_blog_standard
themo_blog_masonry
themo_rooms_standard

Once that is done you will need to reupload or you can use a plugin like https://en-ca.wordpress.org/plugins/regenerate-thumbnails/ to regen the thumbnail sizes. 

I hope that helps you.