That would requier a good ammount of research and study work creating imaginary references from game elements into measurements that can be used to determine the sizes of the ships.
If you want to start at anything, youl have to do some "Avrage size" study of people, and then assume that whichever number on the player characters models. That way you have a good vision of each element inside of ships and their sizes. Use something inside a ship to measure out its length from first person view, or by scoping into another ship. Screen shot it, then calculate using player character as refrence throuout the ship.
Should give you an estimate size.
