Unfortunately you can't in PPT 2003.
Until then, one of Chirag's add-ins should help.
http://officeone.mvps.org/addins.html I think PowerShow is the one you want.

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Echo [MS PPT MVP] http://www.echosvoice.com
Fixing PowerPoint Annoyances http://www.oreilly.com/catalog/powerpointannoy/
PPTLive! Sept 17-20, 2006 http://www.pptlive.com
> How can you increase the font size of your notes on your primary
> monitor while the slide show is showing on the Secondary monitor in a
> dual screen presentation? I hope I asked that right :)