Speaking from experience, it is an extremely complex logic puzzle to allow players to adjust font sizes on the text bubbles unless it's specifically a pixel adjustment that SCALES the font with the text bubble together instead of an actual font size adjustment. The reason being is the systems in place that determine when to advance to the next set of dialogue as well as potential for text being cut off.

To allow for any kind of font size adjustments it would need to be a pixel based scaling which will affect quality, otherwise SE would have to proof read every dialogue in the entire game to make sure nothing is being cut off for various sizes. Huge headache.