In the Med. I use a 75cm with 6.5mm spear and 2X16mm bands around reefs, inside rocks and caves. I change the number of bands used in the shot (1 or 2 16mm bands) depending on the size and distance of the fish I am targeting. I have used it for smaller reef fish, groupers (big ones) inside big holes. If I am going to deeper waters and/or hunting mostly in "open water" around grass, I use a 105cm (I also have a 90cm for in between). In short, you will need at least two different length guns with different power. Eventually, I'll get a 115/110cm gun for strictly open water fish (7 or 7.5mm spear and 3X14mm bands).
In summary, start with a 75cm gun for reef/cave diving to catch reef fish (including big groupers hiding in their holes/caves) and go for a bigger gun 100/105cm later when you are ready and see the need for it.
Note: I haven't been able to catch a dentex with 75cm and had to use the 105 to be able to catch it. For amerberjack, same thing, 105cm. They are too fast and big for a 75cm gun. Groupers are generally stupid and hangout in their dens waiting for you to see them.