Thanks for your comments, Pete.

The label box does adjust automatically, but not gracefully. I was
left with a word whose last letter wrapped to the next line, and
adjusting the font left me with options equally as goofy. I did more
research and found a solution. Thought I'd share for others.
Apparently, in Excel version 2003 the data labels do not expand as in
other Microsoft software. But one can create a text box that can be
sized. To create the text box, click on the pie chart that has been
created, press the "=" key, then point and click the Excel cell that
holds your text. A text box will appear in the middle of your chart,
and it will contain the contents of the original cell (including font
size, etc.) The label box can be moved anywhere and resized.