In Outlook 2003, I use: Tools > Options > Mail Format > Fonts
which brings up font size options for composing a new message, replying and forwarding. If you cannot get there due to the ribbons, there is a third-party program that will bring back the standard menus. These are all options for e-mails you create.
If you are asking about the size of fonts in e-mails that someone else composed, I would not expect Outlook to save your changes to what someone else wrote, unless you change the size when you are replying/forwarding that message. Otherwise, font size will stay the size it was delivered in. You are not saving any changes when you view the e-mail and close it.
It would be nice if you could specify a minimum size font, like I think you can with some Internet browsers, but that option is not available in Outlook 2003.