There is a direct trade off between file size and audio quality, so you may want to experiment with a few different sizes and see how they do. You generally want to go with the highest file size that you can get away with--128 kBit/s is fairly standard for internet use, and that is the highest rate that Cubase will go for MP3.

Another thing to try would be export from Cubase as an uncompressed .wav or .aiff into iTunes and convert to MP3 in iTunes, which can go all the way from 16 kBit/s up to 320 kBit/s so you would have more options. You can also set iTunes to show the file size and bit rate next to the song title (right click in the info bar), so you can convert your song say 5 times, each at a different rate, and have them all showing with their file size--it's a very easy way to see the correlation between bit rate and file size.