I had an issue similar to yours yesterday.
I tried to use the Narrator to read a PDF document (I was feeling lazy and didn't want to read it, big mistake). Adobe Reader apparently has some stuff that integrates with the Ease of Access tools from what I saw. I've never had much luck with this kind of software, but I was really pissed when I saw that the font in the PDF document (unchangeable by man or machine from the original style, as far as I know) was "magically" changed before my eyes into something impossible to read. I quickly closed Narrator hoping that it would go back. Nope. Close and reopen Adobe, same problem. All my PDF's were corrupted by this new font. If I went full screen, I could make it out, but it was still hard to read. I typically snap the document to the right side of the screen (to give room to multitask), and I can still read it just fine. Well, not with this new font that affected every document I had.
So, I used Revo Uninstaller to uninstall Adobe Reader, then I reinstalled it. It worked! Whew!! I was so glad it was back to normal, as I use PDF's all the time for school.
Like I said, I've never had much luck with the Ease of Access tools (Speech Recognition included). Unless you have a top-notch headset mic, and speak in a monotone voice, speech recognition is frustrating at best. As for it changing the font size in emails you type, I can't help but think it has something to do with what happened in Adobe Reader on my computer. You might try uninstalling outlook and reinstalling it (I have no idea if this is possible since it's built into Windows), and not using speech recognition again. I highly doubt you can find a setting that will put the font back to the way it was, as the issue is likely in the speech recognition software itself.
So, I don't really have any good recommendations (besides reinstalling the program). I posted here because I had the same type of issue you have (Ease of Access tools changing font), and I thought it would give you some insight as to what was happening. I would love to help you fix your problem, but unfortunately, I can't.
Best of luck to you, though.