Please can you show me your code for the + button, how do you add an instance? And also can you provide a picture of your hierarchy.
To get a field expandable you must set 3 different properties to make it work fine:
1. Object Palette -> 'Check' Allow Multiple Lines
2. Object Palette -> Field Format -> 'Choose' Rich Text
3. Layout Palette -> Size & Position - > Height -> 'Check' Expand to fit