Try typing 'font size' in the 'Type a question for help' pane, then look for the description which best fits what you're experiencing.
However, it sounds like you've changed the 'zoom view' size somehow. In the Reply view, click the 'Format Text' tab and look for the 'Zoom' icon (magnifying glass symbol). Click on that and you'll see various options - I suggest you click '100%', then 'OK'.