Changing dynamically populated form fields text size

Hello,
we use a software that automatically populate form fields.
We cannot set auto size (just to avoid strings to be truncated when the lenght is bigger than the box size) in the font box, because when we set it to "Auto", the result is "nothing".
 
I tried to use javascript but it seems that the javascript is called before the fields are filled.
 
Thanks for helping
best regards