Automatically adjust font to fit in box when using Data Merge

as far as I know, there are no ways to dynamically change font size via native illustrator tools.

this can be done via a script though. looking at the link provided, it should have what you would need, just need to loop through all the text and check if they are overset, then decrease the font size until they are no longer overset.

  function shrinkFont(textBox) {
    var totalCharCount = textBox.characters.length;
    var lineCount = textBox.lines.length;
    if (textBox.lines.length > 0) {
      var firstLineCharCount = textBox.lines[0].characters.length;
      if (isOverset(textBox, lineCount)) {
        var inc = defaultIncrement;
        while (isOverset(textBox, lineCount)) {
          textBox.textRange.characterAttributes.size -= inc;
        }
      }
    } else if (textBox.characters.length > 0) {
      var inc = defaultIncrement;
      while (isOverset(textBox, lineCount)) {
        textBox.textRange.characterAttributes.size -= inc;
      }
    }
  };

unsure of what you are trying to do via illustrator, but you can dynamically change font size using InDesign and some GREP styled paragraph styles. if you're looking for a code free or code-lite (you'll need to semi understand GREP) method for this, inDesign might be an avenue