First things first, if you want to get more than one line in a field you must make it a multiline field. There is a checkbox on the Object palette that controls this. Second, the field can dynamically grow to fit the content (pushing the content below further down the page). This can be done by making the root subform Flowed and setting the Expand to fit in the Y direction on the Layout palette. When the user fills the field they can enter in as mush text as they want. If they go beyond the visible area the scroll bars will appear while they type the information. Once the user leaves the field then the field will expand to fit the text.
Paul