I don't think AMM's have to be strictly 1 MSP all the time. Early game if you really need decent AMM capabilities you may need the extra space afforded by using size 2 missiles, and later on with high tech it might be worth playing with smaller missiles, eg 0.5 MSP AMM's as submunitions on larger missile buses.
Size 1 is just what is considered the optimum for most circumstances due mostly (I think) to the fact that the smallest missile launcher that you can design is for size 1 missiles, and for AMMs you really want to have that 5s rate of fire.
Outside of use in submunitions, you always want your missiles to have an integer MSP value. Again, this comes from the fact that missile launchers can only be designed to launch integer missile sizes. A size 1.001 missile can only be launched from launchers size 2 and upwards, whereas a similar 1 MSP (exact) missile will have almost identical characteristics but will have improved rate of fire due to being able to be launched from a size 1 launcher.