You can change the font size by clicking on the text, then choosing the resizing tool. There are 5 ready-made options for font size: Tiny, Small, Medium, Large, and Huge, and clicking on one will apply it to your scene:
There is also a slider option you can select, if the 5 ready-made options aren't quite right. The slider lets your adjust the font size gradually, until the text looks perfect:
Different layouts also offer different options, so clicking on the Layouts tab on your scene will give you even more choices for how your scene should look.