Hi Megan,
You don't know how to do this? GASP!
It is very obvious, you see: just right click on the gray scale bar of a slice viewing window, then choose Label, and its size.
To discover other hidden features, watch the change in the cursor as you move it over AFNI windows. It changes its orientation and shape slightly to indicate whether there are right button features there or not.
cheers,
ziad