No, you can't resize hard-coded / burned-in subtitles. You can't alter them at all; they're a part of the video same as grass, trees, and faces. You can't resize any of those.
If your subtitles are changing in size relative to the rest of the video when changing from your PC to being casted, then the subtitles are not hard-coded. They are just inside the mkv container and not a separate subtitle file.