Very interesting - I wonder if some day a lost contemporary picture might emerge of this mystery bird?
I've seen some truly-massive herons, at times. Their size can be very difficult to estimate without proper references.
Interestingly, herons seem (certainly within Scotland) normally to keep well-away from urban & industrial skies, restricting themselves to flights above rivers and protected coastal areas. This gives them something of a localised rarity, if they then do a cross-over into towns.
Some have a blue-grey colouration that makes them almost look like flying shadows, even in full sunlight. Their effortless movement through the air is really impressive, and lends them an immediate other-worldly impression. I've noticed that all other birds (including gulls and ospreys) seem to give herons a wide berth. I recently had the amazing good fortune to watch them feed their nested young (a total first for me) - it looked exactly like I'd imagine a bunfight between pterodactyls.