OP, do you even realize what real space combat could possibly look like? I'm pretty sure you don't, but I can give you a few hints:
1) About design - there wouldn't be such thing as design. At all. Ships would take some of the simplest forms, like cyllinder or sphere.
2) Size - is the relative term. DN ships have the average size of 200-300 meters (destroyers and dreadnoughts) and 50-60 meters for corvette. Is it too big? It depends on the distance to the target.
3) The distance - is the fundametal thing. DN has operating distances up to 12 km. For real space combat this is pretty much point-blank. Even modern weaponry allows to reliably hit targets smaller than 10x10 meters at distances up to 10000 km. Although, it's would be pretty safe thing to do, if we increased those distance to 1 million kms.
4) Now, weaponry - the main criteria - it should be very precise and hit reliably. Is LAZOR precise at 1 million km? Well, more or less. Is it deadly at such range? Not very, as it relatively easy to protect against. But you ask, how about TRUE sci-fi weapons, something like railguns and gauss guns, huh? Well, again, if you want to reliably hit targets at such distances, you need your projectiles to move at relativistic speeds. Anything less than 0.5c would be too easy to dodge. But noone cancelled laws of physics, and Newton's 3d law, right? If you shoot something with SUCH force (and 1 kg slug shot at 0.5s would require ABSOLUTELY IDIOTIC amount of force to accelerate) - this something recoils with EXACTLY the same force. In other words - you either need a HUUUUUGE guns to fire that, or you risk destroying your ship after the 1st shot. Which leaves us only ONE option - good old guided ROCKETS (with nuclear warheads, or what not). Let's assume sci-fi rockets move faster than modern day ones, let's say 1000 km/s. So, for 1 million km fly-time would be around 17 minutes. Cool, right?
Now think about it: how big do you think something of the size of 300m appears from 10000 km? from 1 million km? It's a dot. At best. At worst (and this is most possible scenario) you won't be able to see it at all. This won't be a game of "shoot things". This would be a game of "who spotted whom first". Gameplay would be basically "stare at the radar for 1 hour straight, then press 1 button, then wait for 17-20 minutes, match over, you won". Terrific gaming experience.
Do you really want realism, OP? That's what realism approximately looks like. Do you think it would be a good game? I don't.