Your request seems contradictory. You say that you want a "fixed-size" table and that you want it to expand to accept more data in a field. It can be fixed-size, or it can expand for more data, but not both.

Looking at your document, the table cell expands to accept data but unless there are paragraph marks or line breaks, it would not need to do so. It is already large enough to hold the maximum that you can insert into that formfield. You set it for a maximum of 170 characters. When I increased the maximum to Word's maximum of 32767 characters, I managed to expand your table cell to cover 16 pages, all in that field.