textarea allows to start writing in the middle of the box

The textarea allows me to start writing in the middle of the input field. I set it to allow 500 characters which should be fine for the height I give it in the design.
I have deleted it and created it a new one, but it's still the case.
It works fine if I just leave the design to the default one line height.
As soon as I give it a different height, then the beginning of the writing starts in the middle.
Here's the form: http://assembly21.com/contact.html

Thanks!

Hi,

just use the proper HTML element, i.e. <textarea> instead of the ordinary <input> text field. That said, switch the field type for your message field (see form generator > form settings) to "Textarea" and style this field accordingly.