Any PHPSpellCheck Spell-As-You-Type enabled textarea can be made "elastic" so that it grows vertically to match teh size of it's content.
<textarea name="spellfield"  id="spellfield" cols="50" rows="20" style="height:auto" >
...
</textarea>
<?php 
        require "/phpspellcheck/include.php";
        $mySpell = new SpellAsYouType();
        $mySpell->InstallationPath = "/phpspellcheck/";
        $mySpell->Fields = "ALL";
        echo $mySpell->Activate();
?>