Without knowing which version of IE you're running, I'll assume it's IE6. If the IE | View | Text Size setting doesn't stick, and you're using Eudora 4.2 or greater, then use Tools | Options | Viewing Mail and uncheck "Use Microsoft's Viewer". In this case it's Eudora that's setting the IE text size, and unchecking the above option will stop it. The registry key that Eudora 4.x will change is...
HKEY_CURRENT_USER | Software | Microsoft | Internet Explorer | International | Scripts | 3
IEFontSize=hex:02,00,00,00
If you're running Outlook 2000 or greater... OL | Tools | Options | "Mail Format" tab | "Fonts" button | "International Fonts" button. In the "Fonts" dialog that opens, change the "Font Size" entry to your desired size (for example "Medium", under Western European).