This code is working correct.

<select class="form-select form-select-sm" id="size-cart{{ product.id }}">
{% for key, value in sizes.items %}
{% if key == product.id|slugify %}
  <option selected>{{ value }}</option>
{% endif %}
{% endfor %}

{% for size in all_sizes %}
<option value="{{size.id}}">{{ size.size }}</option>
{% endfor %}
</select>