The high flow cannulas are good up to 20 liters per minute, assuming the rest of the system has the capacity. The nosepiece has a larger diameter body, the tubing is larger in diameter and the fork connector is larger. The flow limitation on a cannula is the tubing that goes over the ears and, to some extent, the two-into-one fork connector. The diameter of the prongs makes no difference to flow.
If you want a special length, just order a 4 or 7-foot cannula and write the length you want in the additional information box.
The length is measured from the fork (2 into 1) connector to the end connector.
