Although the hex distance is purposefully vague, we can get at least a general minimum estimate. A sidebar in the starship section notes that Colossal starships are over 15,000 ft long. Since they can fit in a single hex, a hex is probably at least 15,000 ft long unless you are in a battle where larger ships are unable to interact for some reason.
Like maybe if it was a battle in a debris field and larger ships couldn't reasonably fit in, I suppose.
EDIT: Oh, actually, since a colossal starship could do a flyby of another colossal starship, then maybe the absolute minimum would be 30,000 ft.
