As you build the chart, text might get a little small within the shapes. The Fit Text button helps remedy this.
With the chart selected, click this button on the Org Chart toolbar, and the font size is bumped up a point or two, depending on the available room in the shapes. Sometimes, this is all the adjustment you need.
If you want the text larger than Fit Text has made it, you can always use the Font dialog box (Format menu) to select a font size. But you may need to make the shapes a little bigger, first. Do this through scaling (see the next section).
Note Fit Text can have the effect of making text smaller, also. For example, if some text isn't fitting into a shape, Fit Text will make all the text smaller according to how small it needs to make the text in that one shape. It might be better to scale the chart larger before using Fit Text, in that case.