Chris,
Here are the steps:
1) Open a new form and drop a text field on your form.
2) On the object inspector, click on the "field" tab.
3) Check the "Allow multiple lines" box and set your desired property values.
4) On the "layout" inspector, select "expand to fit" in the dimension that you want (i.e: width and/or height)
5) From the hierrachy, right click on your textfield and select "wrap in subform".
6) Select your new subform that just created. On the "Object" inspector, select "subform" tab.
7) Choose "flow content" for type. Select "top to bottom" for flow direction.
8 ) On the "layout" inspector, select "auto-fit" for both height and width.
9) Now, you can test your form by click on "PDF preview" tab.
Basically, above are the steps that you need. The idea is ,like David in previous post said, put your form field in the "flow content" subform.