https://developer.wordpress.org/reference/functions/get_term_link/
<?php
$args = array( 'hide_empty' => '0');
$categories = get_categories($args);
if($categories){
echo '<ul>';
foreach($categories as $category) {
$link = get_term_link($category);
echo '<li>';
$size = "thumbnail";
$image = get_field('image', 'category_'.$category->term_id);
echo '<img src="' . $image . '" />';
echo '<span class="cat-title"><a href="'.$link.'">' . $category->name . '</a></span>';
echo '</li>';
}
echo '</ul>';
}
?>