Text boxes are not at all flexible. They are essentially graphical objects and should be treated as if they were images. They are not part of the text layer of the underlying document. You can set the wrap to something other than in-line and that will address the text beyond the box. The box will not expand as it is over-filled.
You might do better if you insert a frame instead. This can have a border, wrapping and will expand as configured. Frames are in the text layer of the document.

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Graham Mayor - MS MVP (Word) (2002-2019)
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