What you want is to look at resolution. Small resolution is what we mean by high resolution. A sensor that can see ships from a billion kilometers away is not good at spotting missiles close by. The reverse holds true, that your scanners for finding nearby missiles can't pickup the signatures from ships a billion kilometers away!
You want Resolution 1 scanners for spotting missiles. Lets look at some designs:
Active Sensor Strength: 21 Sensitivity Modifier: 110%
Sensor Size: 1 HS <---this is the size of the sensor, in 50 ton increments. 1HS is too small for a scanner platform, but you should think about putting a small sensor like this on every ship that can shoot-- incase your ship with big sensors gets blown up, you can still shoot! Sensor HTK: 1
Resolution: 1 Maximum Range vs 50 ton object (or larger): 2,310,000 km <-- max range of sensor
Range vs Size 6 Missile (or smaller): 251,559 km <--- this is a good target missile size. aliens seem to prefer size 3-6 missiles, so you want your range here to be big enough that you can see them
Range vs Size 8 Missile: 369,600 km
Range vs Size 12 Missile: 831,600 km
Chance of destruction by electronic damage: 100%
Cost: 21 Crew: 5
Materials Required: 5.25x Duranium 15.75x Uridium
Development Cost for Project: 210RPp
I build sensor ships usually, with great big (size 20-25) sensors-- a big passive thermal, passive electronic, and then res 1 and res 100.
most of my ships get small res 1 sensors, as i wrote above, which lets them continue shooting even if the large sensor ships get blown up. That has happened to me before.