How many megabytes is an mp3?

What, an individual .MP3 song? Depends on two things - the length of the song and the bit rate that the track was ripped at. Obviously ripping a 1 minute song creates a smaller .MP3 file than a 3 minute song at the same bit rate. On the other hand, a 1 minute song at 192kb could be a bigger file than a 3 minute song at 64kb. Higher the bit rate, the better quality the .MP3 will be.