Thank you for your answer Puffer Fish.
So, for you the 5"/8" arms combination is a good idea but it's not effective for animals bigger that 6 feet (around 2 meters) ?
Concerning the effects of the strobes on animals, it is strange to have difficulties for finding informations...
If you mean around 2 meters away, then yes. Flash coverage varies, as would where you have the center of the strobes pointed, so how big of an animal that would cover is pretty variable.
You will see a fair number of absolute opinions of what the "correct" strobe arms lengths should be, but like many things with shooting underwater, the conditions and type of water you shoot in, as well as what type of images you like make a difference.
If, for example, you dive in very clear water all the time, well you don't need long arms. If you dive in hazy water, a little distance helps and if you dive in terrible vis, removing any defuser and getting the strobes out to at least 45 degrees is really important (which can mean more that the standard 5"/8".
I have 3/5/8/12 inch arms and while I normally use the 5/8 combo, have used them all.
If all you are shooting is wide angle, in clear water, you may find that a single arms works.. would not recommend that for general underwater use.
Oddly, about one in of ten dives I have something that can not be lit without actually removing a strobe and holding in my hand. Assuming one can control their position without using their hands, it is kind of fun and it occurs to me that one dive I should just take all the images that way. It is surprising when you do that how many new lighting options come to mind.
Note: The last time was to get an image of a never before photographed deep reef scorpion fish. It was in a recess that the strobe arms made it impossible to get light to. I actually had to hold the strobe in front of and over the top of the camera, farther than the arms would go. The one before that was a leaf fish in a small ledge, where the strobe needed to be under the camera to get any light on it.
The neat part about this is that strobe arms don't cost that much, and regardless of what anyone says, you can try different setups and pick the one you like.