If you're worried you'll have some sci-fi fanatic who has run figures on excel in his bedroom late at night and will declare that your spaceships don't have enough - or too many - raw materials to make them the size they are, just use your own unit of measurement for the materials. A kilgoram is the mass of one litre of water, but where gravity is different, the mass will be different. So perhaps they'd use a unit of volume, or a new universal measurement of mass.
Mass is invariant with environment; it is weight which changes. (Yes, I am one of said fanatics, and not even ashamed of it) Mass is a quality of matter (and energy, actually, in very small quantities); weight is a force, and is better measured in Newtons than kilograms.
In the MKS system litres are a secondary measurement anyway, and the choice of pure water (at 4° Celsius) a questionable standard due to the potential difference in mix of isotopes of both hydrogen an oxygen. Equally, the second has now been divorced from the Earth's mildly variable rotation and defined relative to an atomic resonance.
A long-term space travelling civilisation might well have developed a set of measurements around universal constants – the rest mass of a proton, the speed of light in a vacuum, Planck's constant for all I know – which would ever so gradually trickle down through engineers and thence to the general public, although the old units would be slow in disappearing; you can still, after two hundred years of dismalisation (excuse me, decimalisation), buy a
livre de pain in a French
Boulangerie.
And you haven't considered life support. Any craft inhabited by living beings in space will need an appreciable percentage of its mass taken up with keeping them in this state. Breathable air (and the pipes to get it where it's needed), drinkable water (and likewise) and even, unlike airlines, edible food, and some form of waste disposal/recycling. All mass, and easier to calculate than unknown drive systems (although even there it'll modify a lot depending on whether it's a complete living ecosystem, probably impractical on warships, or nanotechnological molecule separation and resynthesis.
But the drive system is the core of all your calculations. If you're going to accelerate many tonnes of ship up to near light speeds with a conventional drive everything is going to be lightweight, foamed or foil (or even field); the freighter will have no holds, just hookon points for cargo. Make that a gravity-warping device, and everything changes; it might even be useful to have the main skeleton constructed of osmium, to have a more concentrated mass. Synthetic molecules are easier with quadrivalent atoms (carbon, silicon, germanium and apparently tin and lead) rather than the more conventional building metals (here's hoping it doesn't require appreciable quantities of zirconium or ruthenium) You could end up needing quantities of high temperature superconductors; the structure of these is yet to be determined, but it will probably involve at least some of the elements on the previous list.
