Custom Template

I’m looking to change the carousel template to this so that I can use a custom image size, but just changing the template doesn’t make the change permanent. Is there a hook or a place in my child theme where I can place it?

  <div class="slick-image-slide">
  
		<?php
$sliderurl = get_post_meta( get_the_ID(),'wpsisac_slide_link', true );
		echo ($sliderurl !='' ? '<a href="'.$sliderurl.'">' : '');
		if($sliderimage_size == '' || $sliderimage_size == 'medium')  { 
				 the_post_thumbnail('medium'); 			 
			} elseif ($sliderimage_size == 'large') {			
				the_post_thumbnail('large'); 
			} elseif ( $sliderimage_size == 'original') {
				the_post_thumbnail('url'); 
			} elseif ( $sliderimage_size == 'thumbnail') {
				the_post_thumbnail('thumbnail'); 
			} elseif ( $sliderimage_size == 'carousel') {
				the_post_thumbnail('carousel'); 
			}  else {
				 the_post_thumbnail('medium'); 			
			}
		echo ($sliderurl !='' ? '</a>' : '');
	 ?>

	</div>

Thanks,