Ok, I´ll give it a shot
Only talking about offensiv/defensiv missiles, not drones/buoys
Missiles have four main components:
Warhead, engine, fuel, agility
and several "secondary" components:
various sensors, ECM, Armor
If you look at the top-center of the missile design screen, on the current missile technology table, the effects of your current tech are listet, like:
Warhead strength per MSP, Missile Engine Power per MSP and so on.
If your Warhead strength per MSP is, for example, 4, you get a warhead dealing 4 points of damage, if you dedicate 1 Missile Size Point to warhead.
So, not the actual tech is listed, but the effect it has.
You can dedicate fractions of MSPs to each component. AMMs, for example, will often only have 0.01 to 0.04 fuel, as anti-missile-sensors are usualy rather short ranged and so AMMs need only short range too.
Warhead: Defines how much damage your missile will deal to the enemy if it hits. Note: There are no fractional damagepoints.
Engine: The faster your missile goes, the harder is it to shoot down and the higher the chance it can hit the target if it doesn´t get shot down
Fuel: The more fuel it carries, the longer the range
Agility: The more agility a missile has, the more manouverable it is. This increases the chance to hit a target
Sensors on missiles are only used for two situations (that I´m aware of)
a) your missile was shot at a ship, which got destroyed by a previous salvo. If you missile doesn´t mount an on-board sensor, it will simply self destruct, because it no longer has a target. With on-board sensors, it will proceed to the point, its target was seen last and activate its sensor to search for a new target.
b) If the ship launching the missile gets destroyed, the same will happen.
ECM makes it harder for enemy point defense to shoot down your missile
Armor, well, rather self explaining, isn´t it?
Then there is the whole "Multi Stage Missile" thing
You can design something like a MIRV missile
You first design the second stage, say a size 2 missile with range 5mkm, speed 25k, agility 20 and warhead 6 and put 3 of those into a size 12, speed 15k, range 75mkm, agility 10 "carrier missile"
This stuff is, at least partial, explained here:
viewtopic.php?f=19&t=1394