I too had the same problem and tried all the wrong things to recover from the problem, such as repairing, reinstalling etc. The problem did not go away until I reset the zoom size to 100%.
It occurred to me that when working in Word and having saved the file with 'View Whole Page' selected that from time to time Word reopens in this size.
When I thought about the small text problem, it occurred to me that before opening Outlook today, I had saved a Word file in 'View Whole Page' size. When I tried to reply to a message, Word opened at 49%. So for those with this problem, reset the zoom to 100% first before trying the much more complicated stuff!!
Alan