Add Image Size to Advanced Custom Fields

Define the custom size in functions.php

// =========================================================================
// ADD CUSTOM IMAGE SIZE 
// =========================================================================
add_image_size( 'gallery-thumb', 600, 400, array( 'left', 'top' ) ); // Hard crop left top

Single line solution

<?php echo wp_get_attachment_image_src( get_field('field_name'), 'medium' )[0]; ?>

With bootstrap card and variables

 <?php
        $attachment_id = get_field('image');
        $size = 'gallery-thumb';
        $image = wp_get_attachment_image_src( $attachment_id, $size )[0];
       ?>
       
       
        <div class="col-md-3">
            <div class="card">
                <div class="card-img-top">
                    <a href="<?php the_field('link'); ?>">
                        <img src="<?php echo $image; ?>" alt="<?php the_field('title'); ?>">
                    </a>
                </div>
                <div class="card-body">
                    <h4 class="card-title"><?php the_field('title'); ?></h4>
                </div>
            </div>    
        </div>