As far as I know there is no way to change the font size. I believe Handbrake is just burning the subtitle image from the DVD over the video (burning may not be quite the right word, but you get my drift i'm sure) and the font size is set. You could rip without the subtitles if you can find the .srt file for the movie somewhere else on the web..then have the movie and the .srt file in the same folder and most players will use that .srt file to play the subtitles for you.