AMMs need speed an manouverability (agility) to hit anything. The low perfomance nuclear thermal engine needs a loooooot of mass to make your missile fast, so is not realy good for AMMs
AMMs are generally mass 1 (you want your AMM missile launchers to fire as often as possible, and with size-1 launchers you get the fastest reload rate. Also, you´ll need an awful lot of AMMs to defend your fleets, so magazin space (there will never be enough!) is vital. The smaller your AMM, the more you can cram into your mags.
The design process goes something like this:
You design your missile (AMM or ASM)
Then you design a matching launcher (ground or spacebound). Lager missiles need larger launchers with a lower rate of fire and need more magazin space
Then you design a missile targeting system with at least (preferably larger) the range of your missile. The smaller targets can be tracked and the longer the range, the larger the system gets
Then you need an active sensor of preferably larger range, so you can light up the target. As with the targeting system, the longer the range and the smaller the target that can be seen, the larger the sensor gets.
So, while yes, you could design a size-8 AMM, but it would be highly inefficient. For one, if an enemy uses size-4 ASMs with the same tech as you, he will launch 2 salvos in the time, your AMM launchers can shoot only once, giving his every second salvo a free trip to your ships.
I also suggest to have a look into the "Bureau of Ship Design" section on the main page to see some designs
Edit: Damn Steve, would you please stop beating me to an answer!
Just kidding