I really hate what Apple did to the subtitles. At first, the only option was to see the subtitles with the black 50% opacity background.
A couple of updates after, they removed the black background and it was perfect. They where shown in a perfect size, every time.
Now, they allow you to choose how you want the subtitles to appear. It is a nice option but there is a major flaw...
The size of the subtitles change depending on the size of the source... I mean, if I stream a 1280x720p movie that fills the entire screen of the TV they look good at the Medium size. But if the source is, let say, 1280x528p (with black bars up and down), they look tiny and must be changed manually to the Large size.
Also, it seems like the perfect size (something like it was before this update) might be a size between Medium and Large. Medium is not big enough and Large is to big.
I hope it is a bug and they fix it soon.