As with all things in Aurora (but especially missile combat), what you need depends on what your opponent is fielding.
How much range does a missile need? As much range as is necessary for your ships to shoot without getting killed first. If you can shrug off enemy missile fire indefinitely via energy PD, you can put however much range you want and it will be fine. If you need to avoid entering, say, 20 million klicks b/c they will start spamming hundreds of AMMs at you, then you need more than 20million klicks range.
How fast does a missile need to be? Fast enough to get through whatever PD they are using, while balancing the need to include a bit of agility to improve hit chance if you DO get through the PD. Personally, I rarely bother with agility on my attack missiles. Agility's effect is divided by the size of the missile, so it takes a large amount of space to get anything. I usually pick a range and warhead size, then make my missile as fast as possible while meeting those lower bounds. I miss more hits than if I was using the optimum balance of engine and agility, but in exchange I get through PD more easily.
How big a warhead do I need? Balancing this with hit chances is important, but practically speaking you want warhead sizes that are a square number or 1 less than a square. Since a square # will do 1 damage to that armor layer, while 1 off square does 3 damage at its last layer.
This missile still seems pretty bad to me. The warhead is really big, but it's still just so damn slow. Do you not have engine boost tech? I'd be expecting you to reach twice that speed with MP tech.
Also, salvo density is king! 2 launchers is just sad. Try miniaturized, reduced reload rate launchers to get more missiles at once.