IMHO?
The only "realistic" interstellar space vessels that make sense would be captured asteroids utilizing Orion-like propulsion. I haven't looked at the maximum possible mass of an Orion-type spacecraft, but I believe it is substantially above billions of tons if you only have to consider the pusher plate system. Advances in material sciences, and the possibility of "super" systems strengthened utilizing magnetic/electrical charges could dramatically increase this number further, to the point where even the largest of asteroids could potentially be utilized as space craft.
These asteroids would be wired and covered with a variety of useful mounts, including lasers on turrets, a variety of sensors and cameras, railgun-style mass drivers, and a variety of openings protected by plasma windows. On sufficiently large asteroids, these openings could include hangars for auxiliary craft, such as surface to space launchers, and versatile, high-speed drones. Drones could be utilized as scouts, remote sensors, maintenance devices, or perhaps, weapons platforms (suicide or otherwise).
If you needed to militarize such a craft, you wouldn't have to do much. Many of the "tools" on this craft would be versatile enough to be utilized as weapons. A railgun, or sufficiently strong utility laser would be obvious. By virtue of utilizing an asteroid as your "hull", a significant amount of armor is "built in". Turrets/Windows etc. . . could be protected by a variety of means. The above-linked Plasma Window, as well as a variety of Plasma Bubble research suggests to me that the possibilities of creating mixed-phase materials that can be oriented into coherent structures using charges and magnetic fields-- by this I am suggesting a "metal" that retains it shape based on charge passing through, and whose tensile strength is determined by a combination of material properties and energy usage. One can envision clouds of plasma, or even clouds of metals/solids/liquids which could be strengthened utilizing such tools. I would think that these "shield" would not be utilized to protect the entire asteroid, and rather be deployed to protect sensitive portions of the asteroid.
Active countermeasures would be important, as well; railguns/lasers could be utilized to divert the course of incoming projectiles, while electronic countermeasures and radios would be utilized to disrupt/confuse enemy sensors. Boarding "combat" drones could be utilized to attack the propulsion, weapon, and control systems of enemy asteroid-ships; these would probably be launched in swarms, and by railgun.
The "vast" nature of space suggests that there could be two different form of battlegrounds. Interstellar distances are too large to be considered battlegrounds; it only really makes sense to consider solar distances. Inside solar systems, combat between, say, Mars and Earth would be a slow affair; I picture rail guns hurtling projectiles at a significant fraction of light, while defense systems utilizing lasers and smaller projectiles fire back to alter the course of incoming projectiles. At closer scales, combat becomes a more conventional affair, and probably looks like a cross between modern carrier combat and drone warfare.