The simple solution would be to adjust the zoom on your screen. Just hold down the Ctrl button and turn you mouse wheel. It might be that an overnight windows update put everything back to default settings, this is a guess.
The other problem, where the part or all of a message becomes super small, is a glitch that happens when there is a 'hard return' (enter button) during editing the post. The fix for that, if you see it before time out (30 mins) it to go back in, highlight the text, and change the font size to 10, then repost.