This is answered here: Get post thumbnail size
global $_wp_additional_image_sizes;
// Output width
echo $_wp_additional_image_sizes['main_image']['width'];
// Output height
echo $_wp_additional_image_sizes['main_image']['height'];
This is answered here: Get post thumbnail size
global $_wp_additional_image_sizes;
// Output width
echo $_wp_additional_image_sizes['main_image']['width'];
// Output height
echo $_wp_additional_image_sizes['main_image']['height'];