What I have seen in the past was if you make the field expandable and the form dynamic, when the end user types in data, they will be provided with a default input window that they can type into for their comments and if they type more, they will get scroll bars, but as soon as they leave the field, the form will grow to accomodate all of the text. If they enter that field again, they can delete whatever they like. It's close to what you are looking for.
Dave