A function to get with, height, and if crop is available for all image sizes in WordPress regardless if they are built-in sizes or custom set ones

get_images_sizes.php

<?php
// Get information about available image sizes
function get_image_sizes($size = null) {
  $sizes = array();
  $size_data = wp_get_additional_image_sizes();
  $size_names = get_intermediate_image_sizes();
  $default_sizes = array_diff($size_names, array_keys($size_data));

  if ($size) $size_names = array($size);

  foreach($size_names as $size_name) {
    if (in_array($size_name, $default_sizes)) {
      $sizes[$size_name] = array(
        'width'   => get_option($size_name . '_size_w'),
        'height'  => get_option($size_name . '_size_h'),
        'crop'    => (bool) get_option($size_name . '_crop')
      );
    } elseif (isset($size_data[$size_name])) {
      $sizes[$size_name] = array(
        'width'   => $size_data[$size_name]['width'],
        'height'  => $size_data[$size_name]['height'],
        'crop'    => $size_data[$size_name]['crop']
      );
    }
  }

  if ($size) {
    if (isset($sizes[$size])) {
      return $sizes[$size];
    } else {
      return false;
    }
  }

  return $sizes;
}
?>