Size is tiny(from 1kb to 500kb), length also varies (from 10secs to the full song). Generally they are all around 10kb and the full song.
This was from when "polyphonic tones" were all the rage.
Edit:
Regarding the mp3's, must they be from real songs (sure this breaks alot of copyright laws) or custome tracks ?
Edit:
.
.
Nowadays you dont need to download ringtonz, you can easily put the whole track on most phones. Most phones also play mp3/mp4/3gp.