Hi there,
Usually the "Expand to Fit" option under the Height property of the object is what you need.
You also would have to make sure that your text field is "Allow Multiple Lines" & Field Format set to "Rich Text"
Parents objects (subforms & page) are "Allow Page Breaks Within Content" and the Content set to "Flowed"

And once those settings are set make sure that your text field is Expand to Fit on the Height property!

Hope this help!