Instead of a textbox, use a single-cell table with a fixed row height and column width. In current Office versions (I don't recall the exact path in Word 2003 & earlier, but it's mostly the same after the first bit):
• For the rows, set the exact row height under Table Tools>Layout>Properties>Row>Specify Height>Exactly.
• For the columns, set the preferred column width under Table Tools>Layout>Properties>Columns and uncheck the 'automatically resize to fit contents' option under Table Tools>Layout>Properties>Table>Options.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]