Click the Label tab at the top of the Label Editor window.
Click the
arrow tool 
at the top of the editor
window.
Select the text objects.
Handles (little black boxes) will appear around the objects to show they are selected. To select more than one object at a time, click outside of any objects and drag across the objects or hold down the Shift key while clicking each object.
Select the desired font size from the Font Size pop up menu.