Hi there,
There would be much for you to learn on how to expand text fields,
I'll explain briefly and provide you with short videos on how to work with Flowed Forms
First of all, if you want a text field to expand, you should make sure that each subforms / page which are a parent of that text field are Flowed Content
Second, make sure your textfield is Expand to Fit (Height)(You've already done this as I can see)
Third, (if you want your text field to skip to next page if the flowed content is too big for one page) make sure all its parents are Allow Page Breaks Within Content
Allow Page Breaks within Content should also be applied to your text field
Four, set the text field Allow Multiple Lines & Field Format should be Rich Text to avoid any minor errors when treating values
If you have any other questions about LiveCycle, here is a youtube channel which helped me a lot to understand PDF Form with LiveCycle Designer
True Tech Troubleshooting - YouTube
And for videos which will help you in your current issue here is 2 different basics information
Form Flow in Adobe LiveCycle Designer ® - YouTube
Basic Text Fields in Adobe LiveCycle ® - YouTube
Hope this help!