Font size jumps all over in Outlook, on 8.1

[macx]

This is on a friend's PC with 8.1. I don't know much about 8.1 but am trying to help her
fix some issues.

When she's typing an email in Outlook, the font size will jump from huge to tiny or anything
inbetween without her doing anything that she knows.

What's going on and how to stop that?

Thanks!