Like any sport, you can purchase equipment designed for different things.
Spearguns are no exception to this rule, which is why there is such a vast range of them available on the market.
Generally though, they will come under two categories.
The first is air powered, and the second is rubber powered.
Air powered guns are pumped full of air and are usually quite small (around 60cm).
They tend to be used for hunting small fish, especially in shallow water or in caves.
Rubber powered spearguns are much more popular, and these rely on one or more rubber bands to propel the spear forward.
You can get small rubber propelled spearguns, and to make them fire further you simply add more rubbers.
Of course, you need to get a gun that allows for more rubbers if that is what you want.
If you are just starting off with spearfishing, get a small rubber powered gun and learn with that first.
From there, you can decide whether you want to hunt in deep waters off shore, or whether you want to hang around in the shallows.
There are quite a few guns that you can purchase which will do both with relative ease.
By having two rubbers over 16mm you can shoot some pretty decent fish, but the longer the shaft in the gun the better your accuracy will be.
For massive fish you can get huge wooden cannons, but this is only for those with a lot of experience in free diving and spear fishing.