How do I access a custom taxonomy created in Wordpress in Google Tag Manager?

I created a custom taxonomy in WordPress for article lengths. It's called 'Length' and these are the items within it: image

I also created a custom dimension in Google Analytics (it's index 5), and created a custom Data Layer variable in Google Tag Manager for it. But I think I am referencing the Data Layer Variable Name incorrectly and it is 'undefined' when I preview it in gtm.

I can see the taxonomy 'length' and it's classification 'standard' at the end of the class when I inspect the page, but I can't figure out what the Data Layer Variable Name is (I know I need to use dot notation).

Here's the code that shows when I inspect.

<article id="post-2784" class="post post-2784 type-post status-publish format-standard has-post-thumbnail hentry category-deforestacion category-gran-chaco length-standard">
...
</article>

How do I access the taxonomy correctly for gtm? I'm pretty sure it's just this that is incorrect and spent hours debugging and researching but can't figure it out.

I didn't try exactly what was suggested but for those wondering this is what worked for me:

function(){
  
  var length = document.querySelector(".status-publish.post").className.split('length-')[1].split(' ')[0];
 return length; 

}

I had an issue where it would track the dimensions on the homepage too (due to the wordpress theme), so I wrapped it in an if:

    function(){
  var pagePath1 = document.location.pathname.split('?')[0];
  
  if (pagePath1 == '/' ){
} else {
  
  var length = document.querySelector(".status-publish.post").className.split('length-')[1].split(' ')[0];
 return length; 
}
}

It works perfectly with my custom dimensions in Google Analytics and my custom taxonomies in Wordpress!