Fight in a gas cloud.
While cosmic gas clouds are still incredibly thin, at relativistic speeds they will anyway cause considerable friction to your space fighter, so I'd expect more or less the same sorts of maneuvers as in air to be possible. It also means that going faster will cost more energy, and that with your drive off, you'll slow down.
It will not bring the scales down, however, but then larger distances are compensated by larger speeds. There's still the problem of space ships being very small compared to the ranges, but this can be ovwercome by just having a tactical display in your ships that shows the ships much enlarged just to help the intuition of the fighter (because, after all, in the actual space you'll not really see the ship with bare eyes), and weapons that actively steer towards the ships close to their way, so that a shot in the approximate direction (just accurate enough to hit the displayed image if it were not controlled). This would also solve the problem that you don't know exactly where the ship will be when the weapon arrives.
Note that on those distances, lasers would not work because you couldn't sufficiently focus them, and even if you could, aiming them at a far target would be too difficult, especially given that in the time it takes the laser to get there, the ship may already have unpredictably moved to a slightly different place. Also, at large enough distances, also the laser will be weakened by the gas cloud, so its effective range would be reduced. So you'd actually fight with some sort of relativistic missiles. Lasers might be useful for fighting against incoming missiles, however, provided you can detect them sufficiently in advance (which, however, is unlikely unless you're currently retreating in relativistic speed, so the relative speed is low).
So in the end, you'd have a fight that effectively looks similar to a fight in an atmosphere, although part of that similarity is through technical means (enlarged displays, self-steering missiles instead of bullets).