One reason to have very long and seemingly streamlined ships might be they use a mass driver as their propulsion system. This offers some advantages in that the ship can theoretically use almost anything as reaction mass, and if the situation demands, the ship has a built in weapons system.
The excellent Atomic Rockets site contains this set of stats for a mass driver, but the exhaust velocity of 30 km/s isn't quite up to scratch for an interstellar spacecraft, even if you simply strap your craft to a convenient small asteroid to use for fuel. If we were to multiply the length of the drive by ten, according to the equation v2f=v2i+2aΔdvf2=vi2+2aΔd (relativistic effects are negligible at these velocities, so we can ignore them), for a mass driver with the same acceleration, but ten times longer, we get an exhaust velocity of a bit under 95km/s, which is quite a bit nicer, even though we had to increase the mass of the drive and its power source by a factor of ten.
So you can envision the ship being essentially a long "boom" representing the mass driver sticking out the tail of the ship. With large radiators it might even resemble a dragonfly.
Elegant spacecraft design
However, if the ship is undergoing hard maneuvering in space (perhaps it is a warship), then the best design might be similar to a paper airplane. The long "fins" are the radiators, but they work like trusses to keep the mass driver rigid and stable. While paper airplanes usually have 3 fins, a spacecraft is likely to be symmetric and have 4 equally spaced fins to brace the mass driver.
Classic paper airplane. Visualize the fins as the radiators bracing the mass driver in the centre