Royal Knifefish are a separate species, they are Chitala blanci as opposed to common Clown Knifefish which are Chitala ornata.
Royals get to 120cm vs. 100cm for regular Clown Knifes. They are similar care-wise.
I would say you would eventually need a tank with a footprint of at least 8'x3', although I have noticed Royals are quite slow growing compared to regular Clown Knifes and you could probably keep them in a 6'x2'x2' for a couple of years.
Oh yeah, and welcome to MFK. Great to see another Australian member on here.