there are multiple ways to change the sub size, if you use VLC I think the subtitle settings only work for .srt subs, if you want to change .ass attributes the same way get a different video player, otherwise just extract the sub file from the .mkv (using mkvextractgui or whatever), convert it from .ass (or whatever) to .srt and then revert to step one for VLC (tools --> preferences --> subtitles)
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