Return all images sizes register by add_image_size() merged with WordPress default image sizes.

get_image_sizes.php

<?php
/**
 * Return all images sizes register by add_image_size() merged with
 * WordPress default image sizes.
 * @link https://codex.wordpress.org/Function_Reference/get_intermediate_image_sizes
 *
 * @param string $size
 *
 * @return array|bool
 */
function get_image_sizes( $size = '' ) {
	global $_wp_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( $_wp_additional_image_sizes[ $_size ] ) ) {
			$sizes[ $_size ] = array(
				'width'  => $_wp_additional_image_sizes[ $_size ]['width'],
				'height' => $_wp_additional_image_sizes[ $_size ]['height'],
				'crop'   => $_wp_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;
}