You have some options for manipulating your text after the data is populated. Realistically, most such changes are accomplished by using a custom script. For example, a script can detect overset text and size down the font accordingly. What you can do is find an existing script (may be hard) or ask a scripter from Adobe forums or elance to make you one that just reduces font. Then, place that script into your Illustrator/...Presets/.....Scripts/ folder so that it shows up in the File > Scripts menu. Then, this script can be inserted into your action (Insert Menu Item) which batch-processes the datasets. This will run the script on each of your document right after the dataset is populated. This is admittedly a very brief explanation.