Or...you could stop trying to detect missiles with a high resolution sensor and save yourself a lot of space...
Wrong computer so I can't check my just started conventional start game where I have just suceeded in researching the first level of active and em sensor tech, but off the top of my head a single hull space resolution 1 sensor would get you a...you know what, I'm running upstairs to check this out on the computer I wasn't turning on today <.<
Hrm, yes, I forgot how crappy starting tech was
But even with active grav strength 10 and em sensitivity 6, a 250 ton sensor gets you over 300k km...So, working on the theory that your smallest command ship size is one that's capable of jumping it and it's squadron, that would be what, in the 5000-6000 ton range? So your missile intercept ship (because it would be just wacky to try and put all your active sensors for all kinds of use on one ship....right...?) with no need for a jump drive and at the lowest tech would be sacrificing 1/6th of it's hull space for a measly 1.3 MILLION kilometers of size 6 and smaller missile size (sensor size ...tack on a 250 ton fire control with 980 thousand kilometers range and working with 6000 ton ship you end up with
only 4750 tons for engines, launchers, and magazines with a few layers of armour thrown on it just in case. Which can be a good dozen launchers and still be able to keep a good turn of speed. Actually, making the active and fire control match ranges exactly grabs you another 250 tons, which is another 5 magazines which is what, another 80 size 1 intercept missiles
Though, if you go smaller in ship size you kind of start running into problems. I did at one time run a 6k ton command ship with jump engines that had a brood of 3k ton ships. Just meant that instead of a dedicated anti-missile ship I ended up with a dedicated anti-missile sensor ship and a couple of dedicated launch platforms to keep my speed up.