Many Rhoadesii are solds as Yellow Lepturus.
Very true. A very well known importer & distributor in the USA still sells his B. rhoadesii as "Yellow Lepturus". In the juvenile stage you can tell the difference between the rhoadesii & the lepturus by the amount of yellow in the fins/head. Rhoadesii juvies have a fair bit of yellow in the head, where the lepturus will just have a trace of yellow under the chin.
I've read that B. Spectabilis max out at approx 8 inches in the aquarium, but I have only seen juveniles & sub adults so I can't confirm that. What I can confirm is that juvenile males of B. Spectabilis should already be showing signs of adult coloration at 4-5 inches. Male Spectabilis tend to color up at a much smaller size than the other males from this genus. I've seen 4 inch male B. Spectabilis that were already quite blue.
HTH