Ok since I have done fair a bit of work on my ship/missile optimizer, let me share some of my experience.
Engine size/boost: straight forward. Larger engines are more powerful and less fuel-hungry (not significant), and a higher power boost provides more engine power at the cost of significantly more fuel consumption (
http://aurora2.pentarch.org/index.php?topic=8495.msg102804#msg102804 for details). For AMMs and short-ranged ASMs, typically the max available power boost is the way to go.
Note that with the changes in C#, the max power boost tech not only impacts the max power boost available to missiles (of course) but also impacts the fuel consumption when you are not using the max power boost. For example, a missile engine with a 4x power boost will be more fuel-efficient when you have 3x max boost than when you only have 2x max boost. Thus, for missiles especially the ones that need a high boost, the engine boost tech research is more important than before.
Fuel: main parameter you want to tune to adjust a missile's range.
Agility: Together with a missile's speed, it determines the missile's accuracy (i.e., the highest target speed it can hit with 100% chance). The accuracy of a missile can be calculated as:
missile accuracy = missile speed * (10 + agility / missile size in MSP) / 100
where (10 + agility / missile size in MSP) is the maneuver rating of the missile or the 'MR' in the missile description text. Without any agility, a missile can hit a target whose speed is 10% of its own with 100% accuracy. Adding agility is a good way to improve the accuracy of a missile, as long as you don't do it too much. An additional note, the hit chances in the missile design text is also related to this:
missile to hit chance = missile speed / target speed * (10 + agility / missile size in MSP) / 100
Thus the hit chance against a 10kkm/s target is always half of that against a 5kkm/s target.
ECM/ECCM: This is new in C# (see
http://aurora2.pentarch.org/index.php?topic=8495.msg103096#msg103096 for details). In short, ECM on targets will decrease a missile's hit chance, and ECCM installed on a missile can counter that. One thing not explained in that post is that the ECM/ECCM effect is additive in the hit chance calculation. For example, if you have a missile that has 100% accuracy against 12kkm/s targets, and the target with ECM 20 travels at 10kkm/s, then the hit chance will be (12000 / 10000) - 20% = 100%. Assume you can modify that missile and trade some of its speed/agility for ECCM 20, but lower its 100% accuracy to 10kkm/s, then against the same target its hit chance will be (10000 / 10000) - (20%-20%) = 100%.
Sensors: Active/EM/TH sensor equipped missiles have the retargeting ability if their original target is destroyed or lost active contact (although they may not work well/at all in the current version of the game). For retargeting to work, the missiles first travel to the last known location of its original target, then start to use their onboard sensors to try to find new targets to strike. Sensorless missiles will self-destruct immediately when the target is lost. Onboard sensors are typically used for ASMs and mines, not useful at all for AMMs.
Missile design requires carefully balancing the above parameters to achieve (near) optimal performance. In general, for a single-stage missile, you can achieve 2 out of speed, range, accuracy, and damage. Thus, you need to define your missile's role before tweaking any parameters.
For AMMs, a 1-damage warhead is all you need. They should be short-ranged, and as accurate as possible. Usually, a tiny bit of fuel is enough for its range, and the rest of the design process is basically balancing engine size and agility size.
For ASMs, there are many more aspects to consider. They don't need to be as accurate as your AMMs since ships are typically much slower than missiles, but still need some agility to make sure they can hit intended targets with decent accuracy. Faster ASMs are harder for enemy PD to intercept, but their range is usually limited. If you have good ECM tech, you may want to equip them on larger ASMs to make them hard for the enemy to shoot down. Typically, you want an ASM to be as fast as possible while accurate enough to hit your intended target at your intended range.
Once you understand the basics, you can tweak the parameters to your liking somewhat easily. Missile calculators and optimizers can give you some idea of what you can achieve with your current tech. You can check out mine at
http://aurora2.pentarch.org/index.php?topic=10999.0.
