I strongly suggest you only buy a naso if you have the "final" tank already. After seeing this fish in the wild recently - and of course many in public aquaria - it is just one that you can not really appreciate fully in smaller tanks. I personally would say a 6 foot tank in the 200g range is the minimum for keeping a naso, ideally it would be 300g +. The footprint really should be 6 ft minimum IMO.
People will likely argue on your wording. You of course "could" put it in something much smaller and people will say so. But to really see it do well, do start at the 6 foot mark. And they will get from one end to the other in no time at all.
The initial tank, esp for a juvenile, could of course be smaller. But so many people buy them assuming they will get a bigger tank, and do not. This is why I feel the final tank should be a reality and not a dream.