Let's assume the reactor takes up about 4 vertical kilometers and has the same radius as the ship (4km). This gives you a volume of about 2.01e11 m^3, which given about 3-5% of the reactor's estimated mass means your available reaction mass (presumably mostly hydrogen) at any given time would be a few gigatons. This is feasible if you harvest asteroids and comets in interstellar space and could probably last you decades, if not centuries without having to make stops to harvest more fuel (if I'm wrong, please, PLEASE correct me on this). The ship itself weighs around 5.5e18 kg, and I'd like it to be able to pull 1g for an extended period of time (long enough to reach moderate fractions of light speed, up to .1c).
Does my ship's reactor need to be bigger? How much wattage could you get out of a generator this size with this much reaction mass? How long would it last? I really don't know how fusion power compares to fission and fossil fuels in terms of efficiency and energy production, so I could really use some help.