In PowerPoint 2002/XP and above, select the textbox.
Click Slide Show > Custom Animation.
Click Add effect > Emphasis > Grow/Shrink.
Set the size and speed of your preference.
Also, have a look at:
"Tutorial on Retain image quality when Emphasis: Grow"
http://pptheaven.mvps.org/tutorials/ImageEnlarge.html
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Shawn Toh (tohlz)
Microsoft MVP PowerPoint
Site Updated: Sept 26, 2007
(Amazing PowerPoint animations, artworks, games here)
http://pptheaven.mvps.org
PowerPoint Heaven - The Power to Animate