Can't assign random size to digits in SART experiment

Hello I am creating Sart on Psychopy.
I created my condition file and I added size variable in it. I wrote my variable name in letter height part (numberSize). But I receive an error → TypeError: len() of unsized object
Can you help me on this problem ?

Can you explain your problem more thoroughly?
What does your condition file look like? What component and component field are you trying to pass your variable into?