Output the width & height of an image using a particular image style
image_get_info() is the function that will get you the details. But it accepts uri as the param (Not sure about url). You can try passing the argument.
If you manage to get the path of the images then this function return the following ,


"width": Width, in pixels.
"height": Height, in pixels.
"extension": Commonly used file extension for the image.
"mime_type": MIME type ('image/jpeg', 'image/gif', 'image/png').
"file_size": File size in bytes.