Length is cc21. Put data in your track as needed. If you wish to change the default load setting modify the accompanying text file (each instrument has one - same folder) by modifying the second number in the file.
Unfortunately, the electric bass in JABB remains a touch problematic. The new default length (75%) does help prevent held notes from decaying too soon (when the note is held down), but it also creates a long decay even after the the key is released, which makes punchy staccato notes impossible.
What we need is a combination of the two -- short notes should be truly short and cut off almost immediately after the key is released, while long notes should have lots of beefy sustain and not decay too quickly while the key is still being held down. I don't know if you sampled a sharp "mute" release for stacatto notes, where the player uses his right hand to stop the string from vibrating, but that would be nice, especially for high-velocity short notes.