I tried to do that once, I couldnt get anything to work. I have street name signs, and I want the space between words to always be 2 units apart, however, you cant constrain an attribute or text based on its length, only its insertion point, which doesnt help my situation. However, with some complex programming you could probably figure something out, but that level of programming is way over my head, so I decided I would just continue to move the text manually, and not create a block.