In the case you still didn't find out, you can change the properties of textbox with the action textbox -> Set CSS style. On the Property name, write "font-size", and in the Value, you put something and try it out. Right now I'm messing around with trying the value WindowWidht/40 & "px" , but you may choose a permanent size like "30px" if you want. Just try around and see what fits you most.
All the best,