Size of subtitles

I'm now happy I can use subtitles in mkv-files.
But when play the movie in full screen, the subtitle is as small as in no full screen-mode.
The movie gets bigger, but not thew subtitle.
Is there a setting to make the subtitle larger ?
But when play the movie in full screen, the subtitle is as small as in no full screen-mode.
The movie gets bigger, but not thew subtitle.
Is there a setting to make the subtitle larger ?