It seems that the the message you are assigning here is of type "std_msgs/String", have you tried to change the maximum length of the "std_msgs/String" message to match the actual assigned string length?
The screenshot shows the correct pane to set this up, however, I assume it should be the third message (currently it is showing the first message). If this is not the case, please share a model so we can take a look.
Not sure if you've already checked this out, but this example model tries to do a pretty similar work.
Hope this helps.