<script>
function auto_grow(element) {
  element.style.height = "5px";
  element.style.height = (element.scrollHeight) + "px";
}    
</script>
<style>
textarea {
  resize: none;
  overflow: hidden;
  min-height: 50px;
  max-height: 100px;
}    
</style>
<textarea oninput="auto_grow(this)">
</textarea>