Ideal size for a relativistic generation ship?

Assuming the ship has access to fusion power, a large volume of solar panels, and crude antimatter power generators (say maybe 1 gram a day per generator, or whatever is reasonable for technological development a few hundred or a thousand years in the future), how big could a generation ship designed to travel between .1-.99c be? If it's too big, its mass will prohibit it from achieving the desired speeds. If it's too small, it may not have enough living space leftover to accommodate millions of people in at least relative comfort, and it would require less effort to destroy in the event of an incursion by hostile forces (plus it wouldn't look as cool as it would if it were bigger).

Keeping in mind that the ship can be allowed a few weeks, months or even years to reach top speed given the combined power output of the sources listed above, what would be a plausible size for a generation ship that's meant to carry millions as well as defend itself?

posted over 7 years ago

Z.Schroeder‭

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