You don't really want warheads that large. I usually stick to a standard strength-4 warhead for my main ASM from about where you are until around TL 6ish, because warhead strength doesn't matter if your missiles are too slow or unwieldy to actually hit anything. Also note that if you want to be optimal about it, you'll want to make your warheads always be square numbers (1, 4, 9, &c.) due to the way their damage is calculated.
Paul's (I think?) awesome old semi-automated spreadsheet doesn't really work much any more, so I'll lay out the basics.
1. Design your missile drives. Typically you'll want to allocate ~1/4th of the volume of a standard ASM to the drive, and at your TL a good velocity to aim for would probably be somewhere between 16,000 and 22,000 km/s.
2. Set up the rest of your design. Plug in the drive, then make your other allocations.
a) Warhead: Like I said, WH strength 4 is probably the ideal for lower levels. I only bump it up to 9 once my warhead tech is good enough that I'm still using <1.0 MSP for it.
b) Fuel: Not too much, not too little. For the typical ASM you'll want to stick to a range between 125m km - 250m km regardless of your tech level; you'll start out on the low end of that, because your drives burn more fuel and fuel slows you down a lot.
c) Agility: Start off at 1 MSP into agility. Tweak it slightly up and down until you reach the optimal chance to hit. Less is sometimes more.
d) Sensor: I typically just put in a tiny sensor, enough so that if I'm targeting an element of a fleet and destroy it, the rest of the volley can retarget.
Here's an example of a relatively high-tech ASM from my current campaign:
Missile Size: 4 MSP (0.2 HS) Warhead: 9 Armour: 0 Manoeuvre Rating: 42
Speed: 60000 km/s Engine Endurance: 38 minutes Range: 136.0m km
Thermal Sensor Strength: 0.16 Detect Sig Strength 1000: 160,000 km
ECM Level: 0.3
Cost Per Missile: 8.216
Chance to Hit: 1k km/s 2520% 3k km/s 840% 5k km/s 504% 10k km/s 252%
Materials Required: 2.25x Tritanium 0.096x Boronide 0.31x Uridium 5.56x Gallicite Fuel x2500
Development Cost for Project: 822RP
The basic idea is for this to be your standard, which can be used interchangeably on pretty much everything you make. I really like to standardize all of my missile sizes, so that in a pinch my fighters can fire ASMs, my snipers can fire fighter missiles, &c. It also serves as a basis for variation; a fighter missile would take your ASM, bump the WH up a step or two, reduce the fuel, and maybe add a second drive if it works out better that way. A long range missile (which would only come once your warhead tech is pretty good) would reduce the WH, increase the fuel, and maybe beef up the sensor. So on and so forth.
Basically, if you want to use ship-killers with oversized warheads, mount them on your fighters, because they can get close enough without being shot down for you to make the missiles pretty short ranged (25-35m km or so), which helps keep their velocity up and size down.