From a WIP:
If you want even a modest 1:2 thrust ratio in a ship pushing a couple million mass, the engines need to be big. Very big. These put out 1,050,000 thrust versus 1,330,000 mass (and climbing).
Instead of the big circular openings that most science fiction starships have, I wanted my super-titan to have giant bell nozzles that sharply contrasted with the "flying brick" appearance of the rest of the ship, so I came up with this backstory where the engines were salvaged from the drifting hulk of an old colony ship after the factory that produced the engines originally intended for this class of vessel was destroyed by enemy action.