how do i merge mp4 video files using ffmpeg ?
 

I've read man page and FAQs for the same but no success.
Output files contain only the first video but size is combined for all files.
Please help me figure this out!

The (reasonably) hassle-free approach would be to use an interface like Avidemux or Handbrake that takes care of all the gory details for you. If you're after the command line approach then you've got to make all your MPEG-4 parts equal in terms of codecs, aspect ratio et cetera. It's more efficient if you would have posted movie specs, commands used or errors.

i finally used mkvmergeGUI and avidemux to complete the job!