Sawlog lengths are solely dependent on the quality (# defects) and potential veneer or sawlog lengths that can be achieved or maximized when bucking tree-length material to sawlog / veneer log size.
Minimum lengths I've observed on a mill's log deck for black walnut is 7-feet; more common lengths are 8' 6" to 14' 6." YES, log buyers and veneer mills take longer lengths and will definitely pay a PREMIUM for said longer logs.
Therefore, here is my recommendation: If you're felling your own trees and bucking them to length...buck them to the "maximum tree length" possible that way the winning buyer can buck them to their preferred market veneer or sawlog lengths based on the surface defect characteristics of each potential sawlog / veneer log. Many shipping containers are 20-40 feet long, so that is also a consideration for export markets.