Convert mp4 to maximum mobile supported MP4 using FFMPEG

I would like to use ffmpeg to convert an mp4 to 'low size' mp4 ...

I need an mp4 file with h263 video and aac audio (or some other settings supported by low cost mobile.) My main concern is that the video be playable on most devices.

What would be some possible ffmpeg commands to accomplish this?

Thanks in advance.