I want to add ssa subtitles to video.

I've tried with VLC. If I play the video and add subtitles file VLC shows correctly the ssa subtitles but if I convert/save the same video adding the subtitles file, the output doesn't contain the subtitles.

Then I've tried with ffmpeg like this

    ffmpeg -i video.mp4 -vf "ass=mysubtitle.ass" -strict -2 out.mp4

The output shows the subtitles, but the aspect is changed and the subtitles appear partially sometimes since the scale is changed and I would like to get the same aspect ratio (16:9) and same width x height as input video. Or even better reduce video size with same aspect ratio 

How can I do that with ffmpeg, VLC or another open source tool?

Thanks in advance