What is a diplodocus length?

I'm not sure if you're looking for the length at the widest point or an averaged width, but here are the average measurements for the species:

Average measurements (in mm): coracoid length 388.4, femur length 1555.5, femur circumference 560.0, fibula length 1087.0, humerus length 1044.4, humerus circumference 454.8, radius length 795.9, radius circumference 254.5, scapula length 1217.2, scapula width 228.0, tibia length 1035.1, tibia circumference 390.0, ulna length 836.8, ulna circumference 290.0

Source: Paleobiology Database