This is what is known as a kill spike although I call it a shark spike because to me that is it's primary use. Here is a picture of a spike on one of my guns. I made it from a Ray Odor kill spike by cutting off some of the plate, drilling an extra hole and shortening the shaft. If you give Ray the specs I think he'll make one like it, otherwise it will not fit a Riffe speargun.
I think a spike on a speargun in south Florida waters is very important. Sharks show up when the shaft is stuck in a fish. This is a time when a diver is vulnerable as his speargun is little more than a wooden stick. At the least the spike can be used to drive the shark away from the fish on the spear. In the worst case a diver can defend himself from a rushing shark or give an inquisitive one a reason to beware. I put it on all my guns. Recently I modified one of my guns and it became a problem to put one on it. This made me consider carefully where I can connect the spike to the speargun and consequently how long and robust it has to be. These are my thoughts.
No matter how long or sharp a spike is you will NEVER be able to kill a free swimming shark with it, these are the sharks we're concerned about. My experience is that shark skin is very tough yet very sensitive. The sharks react to the poke instantly not giving the tip a chance to sink in. If the spike were able to sink in deep it would not reach any vital organs and it would not hurt the shark any more seriously but it would increases the possibility of the shark ripping the speargun out of your hand. I remember reading an old book where divers would make a shark stick from a wooden dowel by nailing an upside down beer cap to the tip of it.
Consequently for it's intended use I don't think the spike has to be long at all, it need not protrude more than 1.5" in front of the speargun muzzle. This is also long enough to accommodate and detonate a slip-on powerhead.