Use the `minlength` attribute to set a minimum length for the textarea to be valid.

```html
<div class="max-w-[500px]">
  <sd-textarea
    value=""
    size="lg"
    label="Label"
    help-text="Please type at least 5 characters"
    rows="4"
    spellcheck=""
    minlength="5"
  ></sd-textarea>
</div>
```