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

UPDATE1

user@H:/mnt/d/VIDEOS$ ffmpeg -i input.mp4 -vf ass=subtitles.ass -v 40 -strict -2 out.mp4
ffmpeg version 2.8.14-0ubuntu0.16.04.1 Copyright (c) 2000-2018 the FFmpeg developers
  built with gcc 5.4.0 (Ubuntu 5.4.0-6ubuntu1~16.04.9) 20160609
  configuration: --prefix=/usr --extra-version=0ubuntu0.16.04.1 --build-suffix=-ffmpeg --toolchain=hardened --libdir=/usr/lib/x86_64-linux-gnu --incdir=/usr/include/x86_64-linux-gnu --cc=cc --cxx=g++ --enable-gpl --enable-shared --disable-stripping --disable-decoder=libopenjpeg --disable-decoder=libschroedinger --enable-avresample --enable-avisynth --enable-gnutls --enable-ladspa --enable-libass --enable-libbluray --enable-libbs2b --enable-libcaca --enable-libcdio --enable-libflite --enable-libfontconfig --enable-libfreetype --enable-libfribidi --enable-libgme --enable-libgsm --enable-libmodplug --enable-libmp3lame --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-librtmp --enable-libschroedinger --enable-libshine --enable-libsnappy --enable-libsoxr --enable-libspeex --enable-libssh --enable-libtheora --enable-libtwolame --enable-libvorbis --enable-libvpx --enable-libwavpack --enable-libwebp --enable-libx265 --enable-libxvid --enable-libzvbi --enable-openal --enable-opengl --enable-x11grab --enable-libdc1394 --enable-libiec61883 --enable-libzmq --enable-frei0r --enable-libx264 --enable-libopencv
  libavutil      54. 31.100 / 54. 31.100
  libavcodec     56. 60.100 / 56. 60.100
  libavformat    56. 40.101 / 56. 40.101
  libavdevice    56.  4.100 / 56.  4.100
  libavfilter     5. 40.101 /  5. 40.101
  libavresample   2.  1.  0 /  2.  1.  0
  libswscale      3.  1.101 /  3.  1.101
  libswresample   1.  2.101 /  1.  2.101
  libpostproc    53.  3.100 / 53.  3.100
Routing option strict to both codec and muxer layer
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'input.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf56.40.101
  Duration: 00:04:06.06, start: 0.000000, bitrate: 450 kb/s
    Stream #0:0(und): Video: h264 (Main), 3 reference frames (avc1 / 0x31637661), yuv420p(tv, bt709), 854x480 (864x480) [SAR 1:1 DAR 427:240], 314 kb/s, 29.97 fps, 29.97 tbr, 30k tbn, 59.94 tbc (default)
    Metadata:
      handler_name    : VideoHandler
    Stream #0:1(eng): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 128 kb/s (default)
    Metadata:
      handler_name    : SoundHandler
[Parsed_ass_0 @ 0x7f7880] Raster: FreeType 2.6.1
[Parsed_ass_0 @ 0x7f7880] Shaper: FriBidi 0.19.7 (SIMPLE) HarfBuzz-ng 1.0.1 (COMPLEX)
[Parsed_ass_0 @ 0x7f7880] Initialized
[Parsed_ass_0 @ 0x7f7880] Using font provider fontconfig
[Parsed_ass_0 @ 0x7f7880] File size: 3368
[Parsed_ass_0 @ 0x7f7880] [0x826cb0] Style: Default,Goudy Old Style,22,&H002DD8DB,&H000000FF,&H00000000,&H00000000,-1,0,0,0,100,100,0,0,1,2,2,2,10,10,10,1
[Parsed_ass_0 @ 0x7f7880] Added subtitle file: 'subtitles.ass' (2 styles, 27 events)
[graph 0 input from stream 0:0 @ 0x8ed880] w:854 h:480 pixfmt:yuv420p tb:1/30000 fr:30000/1001 sar:1/1 sws_param:flags=2[graph 1 input from stream 0:1 @ 0x8f2ee0] tb:1/44100 samplefmt:fltp samplerate:44100 chlayout:0x3
[libx264 @ 0x82bd00] using SAR=1/1
[libx264 @ 0x82bd00] using cpu capabilities: MMX2 SSE2Fast SSSE3 SSE4.2 AVX FMA3 AVX2 LZCNT BMI2
[libx264 @ 0x82bd00] profile High, level 3.1
[libx264 @ 0x82bd00] 264 - core 148 r2643 5c65704 - H.264/MPEG-4 AVC codec - Copyleft 2003-2015 - http://www.videolan.org/x264.html - options: cabac=1 ref=3 deblock=1:0:0 analyse=0x3:0x113 me=hex subme=7 psy=1 psy_rd=1.00:0.00 mixed_ref=1 me_range=16 chroma_me=1 trellis=1 8x8dct=1 cqm=0 deadzone=21,11 fast_pskip=1 chroma_qp_offset=-2 threads=6 lookahead_threads=1 sliced_threads=0 nr=0 decimate=1 interlaced=0 bluray_compat=0 constrained_intra=0 bframes=3 b_pyramid=2 b_adapt=1 b_bias=0 direct=1 weightb=1 open_gop=0 weightp=2 keyint=250 keyint_min=25 scenecut=40 intra_refresh=0 rc_lookahead=40 rc=crf mbtree=1 crf=23.0 qcomp=0.60 qpmin=0 qpmax=69 qpstep=4 ip_ratio=1.40 aq=1:1.00
Output #0, mp4, to 'out.mp4':
  Metadata:
    major_brand     : isom
    minor_version   : 512
    compatible_brands: isomiso2avc1mp41
    encoder         : Lavf56.40.101
    Stream #0:0(und): Video: h264 (libx264), -1 reference frame ([33][0][0][0] / 0x0021), yuv420p, 854x480 [SAR 1:1 DAR 427:240], q=-1--1, 29.97 fps, 30k tbn, 29.97 tbc (default)
    Metadata:
      handler_name    : VideoHandler
      encoder         : Lavc56.60.100 libx264
    Stream #0:1(eng): Audio: aac ([64][0][0][0] / 0x0040), 44100 Hz, stereo, fltp, 128 kb/s (default)
    Metadata:
      handler_name    : SoundHandler
      encoder         : Lavc56.60.100 aac
Stream mapping:
  Stream #0:0 -> #0:0 (h264 (native) -> h264 (libx264))
  Stream #0:1 -> #0:1 (aac (native) -> aac (native))
Press [q] to stop, [?] for help
[Parsed_ass_0 @ 0x7f7880] Neither PlayResX nor PlayResY defined. Assuming 384x288
[Parsed_ass_0 @ 0x7f7880] fontselect: (Goudy Old Style, 700, 0) -> /usr/share/fonts/truetype/dejavu/DejaVuSans-Bold.ttf, 0, DejaVuSans-Bold
No more output streams to write to, finishing.e=00:04:05.22 bitrate= 562.3kbits/s
frame= 7373 fps=142 q=-1.0 Lsize=   17301kB time=00:04:06.06 bitrate= 576.0kbits/s
video:13086kB audio:3963kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 1.480544%
Input file #0 (input.mp4):
  Input stream #0:0 (video): 7373 packets read (9668740 bytes); 7373 frames decoded;
  Input stream #0:1 (audio): 10597 packets read (3937072 bytes); 10597 frames decoded (10851328 samples);
  Total: 17970 packets (13605812 bytes) demuxed
Output file #0 (out.mp4):
  Output stream #0:0 (video): 7373 frames encoded; 7373 packets muxed (13399929 bytes);
  Output stream #0:1 (audio): 10597 frames encoded (10851328 samples); 10598 packets muxed (4058185 bytes);
  Total: 17971 packets (17458114 bytes) muxed
[libx264 @ 0x82bd00] frame I:38    Avg QP:16.75  size: 22811
[libx264 @ 0x82bd00] frame P:3026  Avg QP:22.01  size:  3818
[libx264 @ 0x82bd00] frame B:4309  Avg QP:24.78  size:   227
[libx264 @ 0x82bd00] consecutive B-frames: 20.0%  5.0%  3.4% 71.6%
[libx264 @ 0x82bd00] mb I  I16..4: 34.7% 50.6% 14.7%
[libx264 @ 0x82bd00] mb P  I16..4:  1.8%  3.4%  0.2%  P16..4: 24.0%  6.6%  5.1%  0.0%  0.0%    skip:59.0%
[libx264 @ 0x82bd00] mb B  I16..4:  0.1%  0.1%  0.0%  B16..8: 14.3%  0.1%  0.0%  direct: 0.3%  skip:85.0%  L0:39.0% L1:59.1% BI: 1.9%
[libx264 @ 0x82bd00] 8x8 transform intra:60.3% inter:53.1%
[libx264 @ 0x82bd00] coded y,uvDC,uvAC intra: 23.0% 35.3% 6.9% inter: 5.4% 5.7% 0.4%
[libx264 @ 0x82bd00] i16 v,h,dc,p: 47% 22% 13% 18%
[libx264 @ 0x82bd00] i8 v,h,dc,ddl,ddr,vr,hd,vl,hu: 36% 16% 34%  2%  2%  2%  2%  2%  2%
[libx264 @ 0x82bd00] i4 v,h,dc,ddl,ddr,vr,hd,vl,hu: 29% 20% 11%  5%  8%  7%  7%  6%  6%
[libx264 @ 0x82bd00] i8c dc,h,v,p: 61% 16% 20%  4%
[libx264 @ 0x82bd00] Weighted P-Frames: Y:17.8% UV:16.1%
[libx264 @ 0x82bd00] ref P L0: 70.2% 11.5% 13.4%  4.9%  0.0%
[libx264 @ 0x82bd00] ref B L0: 91.9%  7.0%  1.1%
[libx264 @ 0x82bd00] ref B L1: 96.6%  3.4%
[libx264 @ 0x82bd00] kb/s:435.73