Here is the code I tested and it works:
<?php
/**
* Plugin Name: Metabox test
*
*/
add_action( 'add_meta_boxes', 'so_custom_meta_box' );
function so_custom_meta_box($post){
add_meta_box('so_meta_box', 'Custom Box', 'custom_element_grid_class_meta_box', $post->post_type, 'normal' , 'high');
}
add_action('save_post', 'so_save_metabox');
function so_save_metabox(){
global $post;
if(isset($_POST["custom_element_grid_class"])){
//UPDATE:
$meta_element_class = $_POST['custom_element_grid_class'];
//END OF UPDATE
update_post_meta($post->ID, 'custom_element_grid_class_meta_box', $meta_element_class);
//print_r($_POST);
}
}
function custom_element_grid_class_meta_box($post){
$meta_element_class = get_post_meta($post->ID, 'custom_element_grid_class_meta_box', true); //true ensures you get just one value instead of an array
?>
<label>Choose the size of the element : </label>
<select name="custom_element_grid_class" id="custom_element_grid_class">
<option value="normal" <?php selected( $meta_element_class, 'normal' ); ?>>normal</option>
<option value="square" <?php selected( $meta_element_class, 'square' ); ?>>square</option>
<option value="wide" <?php selected( $meta_element_class, 'wide' ); ?>>wide</option>
<option value="tall" <?php selected( $meta_element_class, 'tall' ); ?>>tall</option>
</select>
<?php
}