As mentioned, the size of lights is what would determine the size of the locline arms you need. There is a happy medium for size of lights and the amount of light you get. You don't need a zillion watts of lights as even though it throws more light, the further away your subject gets the more backscatter you tend to have.
They also fill the arms with a substance that makes them have more rigidity.
To add more detail, I've used a couple different size lights. The last was a set of two Sola 2000 2000 lumen lights. They were great lights but really overkill most of the time, compared to the cost.
1. How many lights: you must have two lights. Not because of amount of light but because you need to point the lights from two different angles to eliminate shadows. One light would be enough to light your subject, unless really wide angle, but then you would get big shadows in the background.
2. Size of light: for a set of two lights, I think that 1200 lumens of light per light is plenty for day and night shooting. More than that, and I was really overexposing things. If you have a high quality camera with the ability to adjust exposure, etc, then perhaps you could utilize it, but with a GoPro, that was not my experience. You could probably go down to maybe even 600 lumens per light but better would be 800-1000 lumens. That will cover most needs. As for the sheer size of the light, you want as small as possible because this just adds to all the junk you have to lug on vacation no matter how much you love it. The Sola lights are great small size compact lights with lots of output. Fiji where viz is typically clearer than many places but not as clear as it could be, I found I usually had them set to about 50% power. Also note that you don't want to be blasting eveything at night because it just flees. I recently bought iTorch lights and they are a little bigger but still a good light.
3. Hot spot: Extremely important in choosing lights for video is that they should have no hotspot in the center of the beam. You want a nice flat uniform light across the beam of light with no hotspot at all. Otherwise you get overexposure and underxposure going on in the same shot, Essentially you want the light to not be noticeable to the viewer.
4. Battery and charging: some lights have an integrated battery and some have removeable batteries. Consider that a removeable battery can be charged while you are using battery #2 on your current dive. Also consider the size of the charger because it's another thing you have to carry and you need 2. A removeable battery could also be replaced in the future when charging becomes a problem. However, with removeable battery comes the concern that now you have a seal that could flood your light. So pros and cons.
Be certain to thoroughly test anything before going on the big trip. The whole lights and camera set has to have the right relatively neutral buoyancy. Too much or too little and it screws with you the whole dive and makes it very difficult. You still need a red filter for when not using the lights.