Choosing the right wiring duct size depends on the number and diameter of cables you need to accommodate. Measure the total cable bundle size and length required and select a wiring duct that can comfortably hold your cables while leaving some room for future expansion and ventilation. Usually, wiring ducts can be easily cut to the right length with a backsaw (fine-toothed recommended, e.g., hacksaw).
For final distribution on certain levels within the cabinet it is advised to use small cable ducts to optimize space-usage. A good product for this is the tubular, wrap-based HelaDuct flex.