Get image width for a styled image in module
according to documentation, you just need image_style_path instead of image_style_url

and finally 

 $field_image = field_get_items('node', $node, 'field_image', LANGUAGE_NONE);
 $img_uri = file_load($field_image[0]['fid']);
 $img_url = image_style_path('mystyle', $img_uri-&gt;uri);
$img_info = image_get_info ($img_url);