The subform and the form must be set to flowed and dynamic render. On the Layout palette make sure that the Expand to Fit checkbox is checked under the Y: coordinates.
There is a sample that shows this in the Designer install directory. Look under install dir/EN/Samples/Forms/Purchase Order/Dynamic Interactive/Forms/Purchase Order.pdf
The comments field does exactly what you ask.
It shows the scrollbars while editing, but as soon as we tab out of the textfield, it expands the textfield already. Doesn't wait for printing time. Any chance of keeping it in scrollbars up until we actually print it?
If not possible, then I guess this behavior will have to do.