There is no font setting for the Placeholder widget, but you have several options:
Increase the base font size used in the screen. Use this approach if you'd like to increase other font sizes too. To do so, click in the empty space in the editor and then change the "Font" property in Properties view.
Use wiki syntax to increase the font size. You'd use something like this for Placeholder's text: %(font:+2)Placeholder Text%.