get_intermediate_image_sizes()

function get_intermediate_image_sizes() {
	$default_sizes    = array( 'thumbnail', 'medium', 'medium_large', 'large' );
	$additional_sizes = gc_get_additional_image_sizes();

	if ( ! empty( $additional_sizes ) ) {
		$default_sizes = array_merge( $default_sizes, array_keys( $additional_sizes ) );
	}

	/**
	 * Filters the list of intermediate image sizes.
	 *
	 * @since 2.5.0
	 *
	 * @param string[] $default_sizes An array of intermediate image size names. Defaults
	 *                                are 'thumbnail', 'medium', 'medium_large', 'large'.
	 */
	return apply_filters( 'intermediate_image_sizes', $default_sizes );
}

/**
 * Get information about available image sizes
 */
function get_image_sizes( $size = '' ) {
	$gc_additional_image_sizes = gc_get_additional_image_sizes();

	$sizes = array();
	$get_intermediate_image_sizes = get_intermediate_image_sizes();

	// Create the full array with sizes and crop info
	foreach( $get_intermediate_image_sizes as $_size ) {
		if ( in_array( $_size, array( 'thumbnail', 'medium', 'large' ) ) ) {
			$sizes[ $_size ]['width'] = get_option( $_size . '_size_w' );
			$sizes[ $_size ]['height'] = get_option( $_size . '_size_h' );
			$sizes[ $_size ]['crop'] = (bool) get_option( $_size . '_crop' );
		} elseif ( isset( $gc_additional_image_sizes[ $_size ] ) ) {
			$sizes[ $_size ] = array( 
				'width' => $gc_additional_image_sizes[ $_size ]['width'],
				'height' => $gc_additional_image_sizes[ $_size ]['height'],
				'crop' =>  $gc_additional_image_sizes[ $_size ]['crop']
			);
		}
	}

	// Get only 1 size if found
	if ( $size ) {
		if( isset( $sizes[ $size ] ) ) {
			return $sizes[ $size ];
		} else {
			return false;
		}
	}
	return $sizes;
}

var_dump( get_image_sizes() );
/*
array(4) {
  ["thumbnail"]=>
  array(3) {
    ["width"]=>
    string(3) "150"
    ["height"]=>
    string(3) "150"
    ["crop"]=>
    bool(true)
  }
  ["medium"]=>
  array(3) {
    ["width"]=>
    string(3) "300"
    ["height"]=>
    string(3) "300"
    ["crop"]=>
    bool(false)
  }
  ["large"]=>
  array(3) {
    ["width"]=>
    string(4) "1024"
    ["height"]=>
    string(4) "1024"
    ["crop"]=>
    bool(false)
  }
  ["juliobox-size"]=>
  array(3) {
    ["width"]=>
    int(211)
    ["height"]=>
    int(279)
    ["crop"]=>
    bool(false)
  }
}
*/

var_dump( get_image_sizes( 'large' ) );
/*
array(3) {
  ["width"]=>
  int(1024)
  ["height"]=>
  int(1024)
  ["crop"]=>
  bool(false)
}
*/

var_dump( get_image_sizes( 'foo-bar' ) );
/*
bool(false)
*/

$gc_additional_image_sizes = gc_get_additional_image_sizes();

$sizes = array();
$get_intermediate_image_sizes = get_intermediate_image_sizes();

// Create the full array with sizes and crop info
foreach ( $get_intermediate_image_sizes as $_size ) {
  if ( in_array( $_size, array( 'thumbnail', 'medium', 'large' ) ) ) {
    $sizes[ $_size ]['width'] = get_option( $_size . '_size_w' );
    $sizes[ $_size ]['height'] = get_option( $_size . '_size_h' );
    $sizes[ $_size ]['crop'] = (bool) get_option( $_size . '_crop' );
  } elseif ( isset( $gc_additional_image_sizes[ $_size ] ) ) {
    $sizes[ $_size ] = array(
      'width' => $gc_additional_image_sizes[ $_size ]['width'],
      'height' => $gc_additional_image_sizes[ $_size ]['height'],
      'crop' =>  $gc_additional_image_sizes[ $_size ]['crop']
    );
  }
}
foreach ( $sizes as $key => $image_size ) {
  echo  '<li>' . $key . ' ' . $image_size['width'] . ' x ' . $image_size['height'] . ' ' . $image_size['crop'] . '</li>';
}