I'm somewhat unclear as to what the legnth control does. Wouldn't you just control legnth with the midi duration? What does it do?
It controls the release tail. When the midi "note off" is encountered, if there is a release tail as part of the sample, it is sounded at that point. By decreasing the length, you decrease the length of the release tail.
If you want to play very short staccatos, say on the Violins Sus + Short, reducing the length will improve the snappiness of the staccato. This can be varied via CC#21 midi events. 64 is 50%, other values are proportional (127=100%, 0=0%, etc).