Did you check if it isn't your Internet Explorer? Maybe you might have made the text larger by accident or something.
You can check that in View --> Text Size, and be sure it says Medium.
If you have something else, then.. well, honestly I don't know. And if it's not that, then I'm not exactly sure what happened. What I said happens to me sometimes, but since you never mentioned about other text, then I don't know. And I never had large fonts in my sig either.