Using custom image sizes with image/repeater fields

Hi folks,
I've spent many an hour trying to figure this out but I'm still stumped.

I have a bunch of simple slideshows and images set up across a build I'm working on - some just echoing out one image field, some using a repeater to create a slider/showshow gallery.

The issue I'm having is the images uploaded are far too large to display as they are. So, I know I've essentially got to output the ID of the image, not the full URL and resize using the image sizes (much like wp get attachment image src) .

I have set up a bunch of custom image sizes, but I can't figure out how to make this all work. Any advice?

Here are a few examples of what I have so far, which work wonderfully... just need to apply my custom image sizes to these. Sliders are using slidesjs.

    
    <?php if( get_field('images') ): while( has_sub_field('images') ): ?>
        
	    

" />

<?php endwhile; endif; ?>

→



and

<?php while ( $studio_artists->have_posts() ) : $studio_artists->the_post(); ?>
		<?php $rows = get_field('images' ); ?>
		<?php if( $rows ): while( has_sub_field('images') ): ?>
		<?php $artist_name = sanitize_title( $post->post_title ); ?>
				">
					

" />

<?php endwhile; endif; ?>
<?php endwhile; ?>


and

" />


Many many thanks - if anyone would like to help sooner rather than later I'd happily pay a small fee to get this working.
-R