Hell, in a realistic setting, you could make spaceships quite a bit larger than a air-craft carrier of today. Hell, Freighters are bigger. The current biggest was the Seawise Giant oil tanker, that was 458 meters long, and had a displacement of 646,642 tons in full load (Next biggest is under construction, and will be 488m long). Nimitz class is 332 meters long, with a displacement of 106,300 tons. Naval vessels have to take into account the stress of the sea, which has broken many a ship just by wave action alone. Space? Far, far fewer structural stresses. Are kilometer long ships possible? Definitely. Would the engines needed to move them, tear them apart as you suggest? No...Hell, you could strap a model rocket engine onto a ship 1 kilometer long, and it would move it. Very, very slowly, but it would, because engines in space are not so much about speed, but acceleration. Because there is no resistance from particles until you start hitting measurable fractions of relativistic velocity. Until that point, if you could fuel an engine the size of a model rocket for long enough, it would eventually get you going that fast. Because in space, away from gravity wells; there is nothing to impede your acceleration. Hell. Make an engine that is capable of 0.4-1.0g acceleration, suddenly you have artificial gravity all over the ship. So long as you keep accelerating, you have gravity.