"--preset type" type must be: phone, voice, fm, tape, hifi, cd or studio
"--alt-preset type" type must be: "standard", "extreme", "insane", or a value for an average desired bitrate and depending on the value specified, appropriate quality settings will be used.

It all depends on how small you want to go, I did use "--preset voice" on one audiobook and it was noticable when driving. The (female) reader's voice was a bit harsh in places. I don't recall what the final bit-rate was, but the 3 CD took about 46megs of space (I think).

If i was doing it again I would give "--alt-preset type" (where type equalls the targeted bitrate) a go, whether it would be better I don't know.