All the "Victorian" ones I've seen in a few UK cities are rather short. Certainly a lot shorter than modern day versions - but then again I'm not sure how many [if any] were genuine

The tallest one in this set of repros is 11 ft

So, at a rough guess if you go for 50-60mm you probably won't be too far out. Maybe you could even get away with OO model railway lights, judging by the relatively short one 2nd from the left, which I'd guess was about 7-8ft tall.
I'm sure Malamute will have the definitive answer, though
