Is it possible to override subtitle font?

[farruinn]

Sometimes a video selects a specific font and size to use and ignores my settings. My problem is that they are usually not a very legible font or are too small. Is there a way to override this? If not, I've edited external .ssa files, however I don't know how this would be done where the subtitle is part of the video file. (I have already verified that the subtitles are softsubs, not hardsubs.)

[Jean-Baptiste Kempf]

.ssa are hard to edit and force.