Vagelis

Hello there, thanks for letting me know about this.
I will check this and let you know asap.

No you don’t have to set a category, actually, you don’t need to use the shortcode at all if you just need related products on your single product page.

If you want to get 4 columns in the slider to reduce the size of related products you can use the function below into your child theme’s function.php file

function v_woo_related_slider() { 
  if (is_product()) {?>
	<script>
	jQuery(document).ready(function($) {
	$("#woorelatedproducts").data('owlCarousel').destroy();
  	var owl = $("#woorelatedproducts");
	  owl.owlCarousel({
	      items : 4,
	      itemsDesktop : [1000,3],
	      itemsDesktopSmall : [900,3],
	      itemsTablet: [600,2],
	      autoPlay: 3500,
	      itemsMobile : false,
	  });
	  });
	</script> <?php 
	}
}
add_action( 'wp_footer', 'v_woo_related_slider' );

If you are not familiar editing PHP files you should avoid doing that.

if you can’t remove then you should ADD this one

#woorelatedproducts .owl-item {
    max-width: initial!important;
}

This will override your theme’s css

Hello, you mean you need to use related by specific attribute?
Shortcode will work fine with Categories and Tags.

Hello there, I believe it’s your theme’s CSS that causing this issue
could you disable

div.owl-wrapper div.owl-item {
    max-width: 180px;
}

for a moment and test?

Hello thanks, if you have 2 mins a review would be also great.
Thank you.