From the documentation you can use a srt subtitle file and change the size of the font by putting ASS style format KEY=VALUE pairs separated by ,. So,
Code
ffmpeg -i input.mp4 -vf subtitles=sub.srt:force_style='FontName=DejaVu Serif,FontSize=24' -vcodec libx264 -acodec copy -q:v 0 -q:a 0 output.mp4will put the subtitles with DejaVu font and size 24 while keeps the quality of the video. I've tried myself and it worked.
