Also I don't understand how you guys are able to pump so much speed out of missiles and still keep them small.
I think it comes down to the engine power modifier you're using. Unless I want lots of range due to specific reason, I just use maximum power modifier available (minimum 3.5 or I just don't build missiles) add enough fuel until I feel I have reasonable range (50-100 million kilometers for small craft like fighters or gunboats, 100-200 million for warships) and then add warheads and maybe sensors. Here is my version of a long range missile using internal confinement fusion drive (tier 5 drive, the one after magneto-plasma drives) and power modifier 3.75
Missile Size: 6 MSP (0.3 HS) Warhead: 8 Armour: 0 Manoeuvre Rating: 10
Speed: 31200 km/s Engine Endurance: 278 minutes Range: 520.7m km
Cost Per Missile: 4.3438
Chance to Hit: 1k km/s 312% 3k km/s 100% 5k km/s 62.4% 10k km/s 31.2%
Materials Required: 2x Tritanium 2.3438x Gallicite Fuel x4750
The engine size is 2.5, fuel is 1.9 and warhead is 1.6. It should be noted, that the missile is quite slow for the technology and quite large, but made specifically for range due to the doctrine used. As a comparison here is "standard" ranged missiles using size 2 internal confinement fusion drive but with power modified 4.0 (the size is out of whack due to new and not yet implemented reactor technology, it should be size 4)
Missile Size: 3.971 MSP (0.19855 HS) Warhead: 9 Armour: 0 Manoeuvre Rating: 10
Speed: 40300 km/s Engine Endurance: 82 minutes Range: 198.8m km
Active Sensor Strength: 0.297 Sensitivity Modifier: 180%
Resolution: 160 Maximum Range vs 8000 ton object (or larger): 670 000 km
Cost Per Missile: 4.7255
Chance to Hit: 1k km/s 403% 3k km/s 130% 5k km/s 80.6% 10k km/s 40.3%
Materials Required: 2.25x Tritanium 0.1785x Boronide 0.297x Uridium 2x Gallicite Fuel x1967.5
This one has better warhead technology, so engine size is 2, fuel is 0.787, warhead is 0.9 and the rest is sensor and reactor.