Fathoms are usually used for depth.

I'd be more inclined to use leagues, which, when used at sea, in the English-speaking world, are usually three nautical miles or five point five five six kilometers.

So about 5.39 leagues across.

Since we use ships when we go to space and sea, I like the idea that space ships are naval.

When I read the miscorrection of kilometres I was hoping someone would convert the various quantities to fit the oft' overlooked furlong–firkin–fortnight [1] measurement system.

[1] https://en.wikipedia.org/wiki/FFF_system