I've seen darts from 5'-6' or so in length but from what I've read and been told, the length depends on their purpose. Longer darts will fly further, shorter darts may be more acurate at closer ranges. You can also make shorter primary shafts with seperate foreshafts which can be connected to them by use of a sleeve type nock, especially with bamboo darts. Each foreshaft can be specialized for a specific purpose: hunting point, target point, blunt point, etc.). This allows you to carry a few primary shafts and multiple foreshafts when on the trail or hunting. Plus, if the points or foreshaft get broke you only need to replace it; you don't have to make new primary shafts with correct spine.
As for red osier or other wood darts, I'd say cut them to length based on balance point and good spine.