On my work computer, when composing an email in Outlook, suddenly the font size displayed larger than normal. I had this problem one other time, but couldn’t recall how to fix it. The fix is pretty simply.
In Outlook 2007, while composing a new email
Click on Format Text

Outlook 2007 Format Text ribbon bar
Then Zoom

Outlook 2007 Format Text Zoom
Then set the size to 100%.

Outlook 2007 Zoom dialog box
Mine ended up on 200% – not sure how. So I reset it back to 100% and things are back to normal.
And now that I have added it here, when it happens again, maybe I’ll remember how to fix it.
Thanks for this fix – It solved the problem!!!!