There is no simple answer for the right speargun length you need. There are several factors that you will need to consider for the type of diving you are doing. By answering these questions, you can narrow down the right length speargun you will need for the diving you will be doing. Here are some of the important considerations when picking your speargun length. 
The biggest factor in picking your speargun should be normal visibility in your dive location. Visibility is so important, because it will impact fish behavior dramatically. The further away the fish can see a diver the longer the shot that diver may have to take. That means in clean water you will likely need a longer speargun to get the longer ranges. The opposite side of the spectrum is still important to consider. In bad visibility, you will want a short, maneuverable speargun to get the spear pointed at the fish quickly. Even in bad visibility it is important to be able to see the end of your speargun, the fish and what is beyond the fish. Make sure you are practicing safe spearfishing and always know what is beyond your target.
After normal visibility conditions, you need to consider the speargun’s maneuverability. Other than the physical design of the speargun, the length is going to impact how well the speargun tracks through the water. If you are hunting reef fish that dart into holes, ledges or caves, that could mean the right length speargun for you is going to be a short gun. You may not need to take a long shot, but you will need to get your speargun pointed at a fish in a hole quickly, before it hides even deeper in the rocks. Pelagic species, that stay in the water column, may give you a longer opportunity to get the speargun pointed at the fish, so maneuverability may be less important.
The last big consideration comes down to the type of fish you are targeting. Some species just stay far away from divers, so you need a long, powerful speargun to get the range needed to hunt them. Wahoo, tuna, and billfish are all great examples of fish that usually require longer shots. There are always stories about divers who were lucky enough to get point blank shots but, overall, these fish keep their distance. That means long, powerful spearguns with four to six bands are needed. That size gun loses maneuverability and really can’t be used in limited visibility, but it is a specialized speargun for a specific purpose. Most reef species don’t need as much speargun, but it can come down to your hunting technique.
You should get a speargun that has a similar effective range to the visibility where you dive. The effective range of a speargun is, generally, about two to three times the length of the speargun’s band stretch distance. Notable exceptions come into play if you are targeting reef species in holes–then you just want a speargun that has enough power to put the spear shaft through the fish. The challenge of picking the right speargun comes down to the problem that there is no single speargun that meets all of a diver’s needs. As time goes by you will collect all the spearguns you need for the types of diving you do.