My font size keeps changing when I write emails!

This happens every day, seemingly randomly.  I constantly have to write the email, highlight it all, and set the desired font and font size. This happens despite the fact that I (more than once) have gone into Options and set a default.

Also, sometimes the email will appear to be fine, so I send it. Then, I will see it after it's been sent, and there are 2 different fonts in it. This is driving me crazy, can someone please advise?