Hi Patti,
You can get this functionaliy by adding formulas in the Text Transform
section of the ShapeSheet for the text block. In order to get this
functionality, the formulas for the TxtWidth and TxtHeight cells can be set
as follows:
TxtWidth: TEXTWIDTH(TheText)
TxtHeight: TEXTHEIGHT(TheText, TxtWidth)
This will cause Visio to dynamically resize the text block as text is
entered or when text is set via solution code or other automation. In order
for new instances of the master shape to have these formulas, the master
itself needs to be edited.