re: size estimates
Going along for the sake of argument with the sighting being as described, the lettering for the type of safety markings he described would be a standard size, often described in military specification down to the fraction of an inch. The escape hatch markings on a jet fighter aren't that much different in size than those on a large bomber.
In fact, comparing these markings to the rest of the ship might be the most effective way of calculating the size of the vehicle.