Does this only happen when you go to the email site? All other I.E. windows are fine? After you maximize it, you get the full screen, can use it, then how does it go back to the small size? On it's own?
I don't see a single web site having this issues when others are fine, something is going on here that's not quite as described.
You can try to clear temp files and cookies, keep in mind that will clear your saved passwords and maybe some other site settings.