WooCommerce Simple Product Options List

You can use product attributes. You can configure them in the back-end, and then add something like this to your template:

<?php $sizes = $product->get_attribute( 'size' ); $sizes = explode(", ",$sizes); ?>
<?php if($sizes[0]!=''){ // if product sizes are available ?>
  <span class="product-size-label">Choose your size:</span>
  <select class="product-size">
  <?php foreach($sizes as $size){ ?>
    <option><?php echo $size; ?></option>
  <?php } ?>
  </select>
<?php } ?>

However, if it affects the price or appearance of your products, you have to use product variations. That’s what they were designed for. As far as plugins go, I haven’t tried any of them but it looks like there’s a few that could achieve that fucntionality, e.g. this one.