We have a user that is seeing this intermittent problem where
periodically when he sends a message the font size on only the body
text changes from 10 to 12. The size of the greeting and closing
remain at 10.

He is using the default font size which is 10 for all message types.
Word as email editor is off. The text in his emails is what is
spontaneously changing here. He just typed the message, does no any
additional formatting, and no signature is used. He sent the message,
but when he received the reply, he noticed that in his original the
body text size was larger. He checked the message in his sent items,
and the body text is larger there too, but when he sent the message
everything was the same size, 10.