Reduces initial height of Advanced Custom Fields WYSIWYG fields to 100px, and enables autoresizing of WYSIWYG field, as text is entered. Best practice would be to include this function in a site-specific plugin.

acf_modifications.php

<?php

/*
 * -----------------------------------------------------------------------------
 * Advanced Custom Fields Modifications
 * -----------------------------------------------------------------------------
*/
function PREFIX_apply_acf_modifications() {
?>
  <style>
    .acf-editor-wrap iframe {
      min-height: 0;
    }
  </style>
  <script>
    (function($) {
      // (filter called before the tinyMCE instance is created)
      acf.add_filter('wysiwyg_tinymce_settings', function(mceInit, id, $field) {
        // enable autoresizing of the WYSIWYG editor
        mceInit.wp_autoresize_on = true;
        return mceInit;
      });
      // (action called when a WYSIWYG tinymce element has been initialized)
      acf.add_action('wysiwyg_tinymce_init', function(ed, id, mceInit, $field) {
        // reduce tinymce's min-height settings
        ed.settings.autoresize_min_height = 100;
        // reduce iframe's 'height' style to match tinymce settings
        $('.acf-editor-wrap iframe').css('height', '100px');
      });
    })(jQuery)
  </script>
<?php
}

/*
 * -----------------------------------------------------------------------------
 * WordPress hooks
 * -----------------------------------------------------------------------------
*/
add_action('acf/input/admin_footer', 'PREFIX_apply_acf_modifications');

Awesome, much appreciated!

Since I make heavy use of the 'delay'-option ("Defers initialization of editor until editor is clicked instead of on page load. This is useful to speed up load times") I added a line that targets the initial Textarea accrodingly – the small height is compensated by a vertical scroll and on init the TinyMCE-box automatically resizes so no downsides there IMHO...

This is my modified Script:

<?php

/*
 * -----------------------------------------------------------------------------
 * Advanced Custom Fields Modifications
 * -----------------------------------------------------------------------------
*/
function PREFIX_apply_acf_modifications() {
?>
  <style>
    .acf-editor-wrap iframe {
      min-height: 0;
    }
  </style>
  <script>
    (function($) {
      // reduce placeholder textarea height to match tinymce settings (when using delay-setting)
      $('.acf-editor-wrap.delay textarea').css('height', '100px');
      // (filter called before the tinyMCE instance is created)
      acf.add_filter('wysiwyg_tinymce_settings', function(mceInit, id, $field) {
        // enable autoresizing of the WYSIWYG editor
        mceInit.wp_autoresize_on = true;
        return mceInit;
      });
      // (action called when a WYSIWYG tinymce element has been initialized)
      acf.add_action('wysiwyg_tinymce_init', function(ed, id, mceInit, $field) {
        // reduce tinymce's min-height settings
        ed.settings.autoresize_min_height = 100;
        // reduce iframe's 'height' style to match tinymce settings
        $('.acf-editor-wrap iframe').css('height', '100px');
      });
    })(jQuery)
  </script>
<?php
}

/*
 * -----------------------------------------------------------------------------
 * WordPress hooks
 * -----------------------------------------------------------------------------
*/
add_action('acf/input/admin_footer', 'PREFIX_apply_acf_modifications');