@rezingg: I created a simple shape, basically, as Vojo describes. And, that behaves as one would expect. The width remains constant regardless of how much text is entered. Visio automatically wraps the text and grows the height. This is standard behavior. Note, the actual boundaries defining the shape do not change. The shape can be manually expanded in width and the wrapping will adjust to new width.
Out of curiosity, I modified the TxtLocPinY cell as per your description. Had no impact upon behavior. All above remain unchanged.
So, if the behavior you're seeing doesn't match the above, perhaps you can upload an example. If otherwise, I couldn't replicate any abnormal behavior.