Auto TextSize Container?

Hello,
 
I am designing a form that will use a text field we will call "A". Text field "A" is set to auto-size the text string (textSize = 0). I need another read only text field "B" that dynamically displays the text size in points of the string entered in text field "A". I should be able to use a keystroke event to capture the data. However I do not know where the data is being stored. It seems that if I make a call on f.textsize it will return 0 no matter what the actual textsize is due to the autoformatting based on bounding box size.
 
Thank You,
Robert Laughman