tommy -
The scale length is the nominal length from the nut to the bridge. It sounds like the 21 you are measuring is the neck length which isn't really very meaningful (it is what you said you measured in your original post). As a checkpoint, the scale length of a Stratocaster is 25.5, so a 21 scale bass is small indeed!
The easiest way to measure the scale length is to measure from the nut to the center of the 12th fret wire and double that dimension. The reason that you measure it this way is that a properly set intonation adjustment at the bridge will measure longer than the true scale.
For what it's worth, I'm pretty sure that a Mustang bass IS shorter than 30.75 - 29 seems to stick in my head.
If you want the smallest overall length, the small body, Stanley shape will probably be the shortest. Many of the Alembic body styles put the bridge closer to the middle of the body than a Fender bass. Since there's a separate tailpiece as well, that's at least a couple of additional inches of body width. The problem with that body shape is that it tends to balance poorly because of the very short cutaways. If you had a custom built, perhaps you can have them do a special placement of the bridge and tailpiece to reduce overall length.
David Fung