The Short Answer: We don't know until a space war breaks out
The long Answer: We could guess,right?
On the side note, The Expanse presents a not bad representation of a spacecraft built for war
For Real Life:
Size: It depends on whether there is a crew or not. If there is a crew, then the spacecraft will be designed to have life support and crew living quarters and ample amount of space for maximum crew comfort. A unmanned spacecraft would be even smaller, probably equivalent to satellite, of course with weapons. You could even go smaller to nano meter sizes. Think of swarms of nano machines in space fighting other nano machines or destroying whole planets. It would be more logical to have a unmanned spacecraft to fight our wars.
I'am also thinking of hitting planets with projectiles accelerated to a near the speed of light instead of having spacecraft fight in space.
Its important to know that space is not a ocean.
Space fighters will probably be space planes firing anti-spacecraft missiles, they probably will be on a suborbital trajectory. Its very likely they would be unmanned,
Weapons: It depends on the time period, in the cold war. The USAF has a plan for a Orion nuclear spacecraft armed with a nuclear warhead and 3 inch naval guns for self-defense. People mostly think of lasers and rail guns to be on spacecrafts in war. Why not include a traditional naval gun probably with ECT(Electro-Chemical Technology) and other improvements? Something like the CIWS would be important on spacecraft for shooting down missiles or space fighters. I think of two types of missiles: One for close range and for long range. The close range would be something like the AIM-9 sidewinder that launch in close range. A long-range missiles would be like a probe or a nuclear missile that target other spacecraft far away or not in reach.
Appearance: Think of a NASA proposed Orion nuclear powered spacecraft or Constellation Mars transfer vehicle combined with a tank. Its very unlikely, they would be built like ships on Earth. For projectile protection; something like a wipple shield or spaced armor to stop rail gun rounds or angular armor like a tank to increase thickness and chance of round ricochet.