How are you measuring the size of the mp3 file? Using minutes and seconds alone isn't going to work. Bitrate determines how large your file is. For example, a 19 sec song sample at 128kb (bitrate) is much larger than the same 19 sec song sample at 94kb. 190kb is cd quality, 128 (which I use) is still very good, but not pristine. 94 kb is...crap actually. The thing is, you don't need cd quality for a ringtone. Convert the song file you want to use to a lower bitrate and you'll have no problem.
"You can't make sense out of nonsense."