Tuneskit converted movie subtitle is small

Hello! I need help with tuneskit. when i have converted a movie i have bought from itunes the text is barely visible. i have tried using different movie players such as vlc, plex, windows media player so on. But the only one that works is quick time player. I kind of need that the text works with plex because i share all my media with it. Any suggestions?
thanks on forehand ;-)