WordPress size images

If the image you are interested in is attached to a post by using the attachment post_type (the normal situation), you can get all sorts of meta data, including pixel sizes of each defined size (thumbnail, medium, large, etc.), with wp_get_attachment_metadata().

You will need the attachment post’s ID, which can be found various ways. For example, getting all image attachments whose parent ID is that of the current post. Then check the corresponding filenames until the correct attachment post is found.

I see, thanks for the extra info. AFAICT it appears to be a server configuration or security issue for smartcommunity.es. I also am unable to use getimagesize() on images on that server. I get:

Warning: getimagesize(http://smartcommunity.es/wp-content/uploads/2017/03/yyyy.png): failed to open stream: HTTP request failed! HTTP/1.1 403 Forbidden

The images there open fine in a normal browser. In saving the image, then uploading to my server by FTP, getimagesize() works fine, so it is not anything related to the image itself or WP in general. It appears to be specific to the smartcommunity.es server where file access via PHP is forbidden there.