The spiked chain does say that it provides 10’ reach while retaining the ability to attack adjacent foes. It mentions sizing, as well (the 10’ reach is a medium sized weapon, I assume). It does not say how long a spiked chain for a medium user is, though. What would happen if five feet of length were added? Would that resize the chain inappropriately? It isn’t enough length to provide the right size for a large user, is it (that requires an extra 10 feet, seemingly)? Or does that length (whatever it is) turn out to be a useless one? The whip (again, whatever length it actually is) provides 15 feet of reach without threatening. Perhaps there is some room for imagining a slightly longer spiked chain. I guess that would count as a house rule, though, as it certainly isn’t mentioned.