Use the smart paste tag that pops up when you paste. Select "keep source formatting," and it should set things right.
Unfortunately, you can't set the default behavior for paste.

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Echo [MS PPT MVP]
http://www.echosvoice.com
presenter, PPT Live '04
Oct 10-13, San Diego http://www.powerpointlive.com
> Whenever we copy text from one slide at any font size,
> when we paste it into a new slide it keeps changing the
> font size. I thought it was the autofit function, but we
> unchecked the box for that and it still does it. Any
> ideas? We are using PowerPoint 2002.