Creo Schematics does not have any way to know how a cable will be routed in 3D. It has no information on wire lengths.
By "define the length", do you mean to force a cable to a specific length when routed, or to report the length of the cable after it is routed.
Forcing a specific length (like when you use a harness in multiple systems) is done by trial and error in Creo Parametric. We add extra bends for short items or shift the routing points. If a harness is long enough, we use a loop. One of our people created an object that coils of wire can be linked to and has a variable diameter. The diameter of the loop can be calculated from the amount of wire that is not accounted for by the normal routing of the wire.
For getting the length of wires in Schematics, you need to route the harness and save a report in Creo Parametric. Then you import the saved report back into Schematics and place it on a page.
The link between Schematics and Parametric is one way.