This site will look much better in a browser that supports web standards, but it is accessible to any browser or Internet device.
Font Changes Size in Internet Explorer 5 Using Eudora Pro 4.1 or 4.2
Font Size Change Is Not Saved in Internet Explorer 5
Font Size Settings Not Retained in Internet Explorer and Outlook Express
If you are using Outlook 2000, go to tools -> options -> mail format -> fonts -> international fonts and chance each font size entry to your preference.
If you do not use Eudora, and the other suggestions do not apply, perhaps MSHTML.DLL is corrupt or not properly registered. Go to start / run and type:
If still no joy, use System File Checker (SFC.EXE) to extract a fresh copy from c:\windows update setup files, or if using Windows XP, run MSCONFIG, then click on the Expand File button.