highlight the text you want changed and look around on the top toolbar for some numbers, then, click the arrow next to it and press (or put in your own) a number. Ta-dah font changed!
Depend where you are. Word processors all have specific commands (look for FONT) In Windows main screen you can change the properties of the screen. (right click on a blank part of the screen).
What i would try is observing her hand in an unsneaky way so she wont get suspicious. Then if her fingers are just about your size or a little bigger try and estimate the correct size =D. And if that...