First of all Auto FOnt is a feature which is specific to Acrobat, it shouldn't be used in designer.
However since the functionality is exposed a lot of users do that.
When AutoFont is used, the font takes as much size as it can , with in the text field, thus growing from default 10 to 12.
only way to regulate this is
1. initially do not use the Autofont.
2. Get a font which has all letters of equal width i.e. a font where W or I would take same amount of horizontal space.
3. once you have decided one such font, you can count the no. of letters that can fit in the text field with font size 10.
4. let's say your text field can fit 10 characters before it will need to reduce the font size, so you can apply a script now, which calculates the no. of characters in the text field , this will have to calculated at every key stroke. Whenever the no. of characters goes beyond 10, set the font size of text field = 0 (which means auto font), and when someone deletes the characters, and count comes below 10, again set the font size for text field as 10.
trust me there's no other way to do this.
~Sid
