Just for the others out there, the answer is not to use an Input box as this only allows single lines.

You need to use a TEXTAREA which allows you to specify the number of rows and columns for you text box and any default text in the box:

Code:
<textarea cols="40" rows="6">
Type your reply here...
</textarea>
You can always drop in some JavaScript code to select all the text when the textarea box gets the focus.