The Ceres-class Supertanker is one of the largest bulk haulers in known space. It is half a mile long, filled with massive cargo hold space and has a 2,000,000 DWT (deadweight ton) cargo capacity.
Moving all that mass requires four large helium-3 fueled fusion reactors, and even then the ship can only attain about half the superluminal speed of military vessels.
The ships are unarmed, but are rarely endangered by pirates or raiders because they tend to go directly from their departure station to their arrival station with as little sublight travel as possible. The only time they are truly targets is during open war, in which case they are often accompanied by frigate escorts.
A couple of independent planets have bought these freighters, given them light anti-starfighter guns and turned them into massive carriers. While they can contain hundreds of fighters, their lack of speed, maneuverability and advanced sensors make their combat value questionable.
I modeled this a touch after real supertankers. You can see four massive airlocks on top for bulk cargo transfer. Small transports can even land in them. There are two bridges. At the front is the "wheelhouse" for navigation and flight controls. Then in the back, overlooking the main body of the vessel, is a second bridge where cargo operations are overseen. That bridge is actually more important than the flight bridge, since the course of these ships is set before they even leave orbit. All the pilot has to do is navigate the ship through orbital traffic and out of the gravity well. Besides that, they're just there in case of emergencies.
Enjoy, and let me know what you think.