I was recently pointed to this site, showcasing a variety of different starships and spacestations (and apparently people and creatures) from science fiction, all to scale; from the tiny Tardis to the daunting Dyson Sphere:
Click on the second to largest scale, "-2000X". This page is arranged in descending order of size. At the top, at 10,000 km across, is Halo. Well, about half of it; and in the background is maybe a quarter of the Earth's moon. At the very bottom, for no particular reason it seems, is the Martian moon Phobos, at 27 km x 23 km x 20 km. Deimos, from which the Marathon was made, was even smaller than that - according to Wikipedia, only 15 km × 12 km × 10 km. For comparison, the Death Star (which was the first comparable object that came to my mind when I first heard of the Marathon) is 120-160km across (depending on which version, I or II).
So it seems our dear Marathon, which all along I had pictured as this massive behemoth of a moon, not as big as our own but still on roughly the same scale, is smaller across at it's widest than the city I live in. The Pfhor scoutship was over 1/5 the length of the Marathon, by these figures.
Kind of disappointing :-(