I had the same issue, and found that it reverts back to regular size if I go to the calendar page, then close outlook, then reopen. Somehow this resets the reminder box to normal size.
I tried this because a few months ago I had an issue with Outlook all the sudden making my font extremely tiny. One of the forums suggested going to the calendar, then closing outlook, then reopening and font reverts back to regular size (which works!). I still get this bug occasionally too, but until Microsoft fixes these ridiculous issues, at least going to Calendar, closing, opening seems to somehow refresh outlook. Hope that helps!