No Human has been inside a Citizen vessel - or at least done so and returned to tell about it. However, since most of the fleet stays in either high earth or lunar orbit, they can be seen easily at night - as moving lights to the naked eye or clear structures through a telescope. Currently there are at least twenty smaller vessels and five larger ones present, though initial reports upon arrival announced up to forty units, and only eight were confirmed destroyed.
Alien ships from any given SF movie are far more odd than what the PC vessels actually look like. Quite frankly, they look like large bricks wedged in the center of a skeletal hourglass. The interior block is the general useful area of the ship - living quarters, command centers hydroponics and so forth. Reactors are held away on hundred meter booms with interposed shadow shields for safety. Latticework structures surrounding the fulfill three important functions. They act as radiators for reactor and living quarter's waste heat, electromagnetic field generators to deflect charged particles and small debris (thus keeping the central area safe, and magnetic thrust nozzles for the plasma based drives.
Even the smaller vessels are huge, with the central habitat comparable to an aircraft carrier in size, with their EM generators stretching about three times that distance further. In numerical terms - approximately 340 meters long, 50 meters wide and high - though the extended "tail" with shadow-shield and reactor adds another 60 meters or so. The Lattice would then extend nearly a kilometer in total length - the midpoint attached to the mid point of the habitat. Larger vessels might be two and a half times these figures.
The outer surface of the ships seems to be a light gray, though close analysis reveals they were white but suffer extensive pitting and wear, reducing the albedo. Meanwhile, the metal lattice seems to be polished silver, though often shows shimmering colors when the plasma jets are in action or charged particles are captured - just like the Aurora Borealis. (Indeed, its the exact same effect, just on a smaller scale.)
Aside from the complications of sheer scale most of the technology of the ships is within human understanding. Human Variable Specific Impulse Magnetoplasma Rocket (VASIMR) propulsion units are quite similar to the aliens, just far smaller. Analysis of destroyed remnants reveal the ships are made from high strength steel alloys displaying a crystalline internal structure which in turn implies the metals were smelted and forged in micro-gravity. These are backed by other materials to provide insulation, radiation protection, and crew comfort.
The need to isolate the reactors from the crew areas by several dozen meters would imply that they are fission units with dangerous levels of output. Fortunately, none of these power sources have crashed to earth - the aliens have taken great care in keeping them safe, even when the rest of the ship is compromised.
Of course, the most outstanding element would be the Citizen faster than light mechanism. Little is known about it, even by the aliens that have been interrogated so far.
The ships are seen to rotate on their longitudinal axis, implying that they lack artificial gravity and thus must simulate it centripetally.Following such logic, educated guessing would place living and recreational areas in the outer protions of the ship, while machinery and storage that doesn't suffer ill health in micro-g would be closer to the core. It is unknown if the very center holds a protected storm shelter to mitigate cosmic rays for the crew, or if the ships EM generators can deflect solar flares, eliminating the need for such precautions.
At the extreme front of the vessels is a loading bay for docked sub-craft (shuttles and maintenance units) as trying to leave by a side exit would throw them into the EM lattice work. There are no exits at the other end due to the dangerous reactors.
Population density of the vessels is still an unknown. Some PCs have divulged that the ships are designed to operate for generations in event of a drive failure and to allow the creation of new colonies - thus implying up to thousands of people - crew and civilian aboard. Low Range estimates for the fleet's population is around 600,000 while four times that is considered the upper range- not including the possibility of either cloning or cryogenic storage of colonists.