Anything can be done. I guess you could change the height property of the box
depending on how much text is in it. You would have to calculate the number
of lines in the box based on the total number of characters in the text
divided by the average or approximate number of characters available on each
line, then resize each time there is enough additional text to warrant a
resize.
Not much fun, though.
As noted, the CanGrow property of a control is effective only when the form
is printed or previewed, not at edit/input time.