There is definitely a broad spectrum of hotels that fit within the category of "luxury." It is definitely possible to have a very luxurious experience at a larger hotel, especially if one is a regular or suite guest and receives a particularly high level of service.

If we measure luxury by the baseline level of service the average guest will experience, booking standard accommodation categories and without any guest history/profile, I think it is quite clear that luxury is inversely correlated with room count. An example that immediately comes to mind is Four Seasons Marunouchi, with its 57 rooms, every single guest is guaranteed a high level of service. In my experience objectively luxurious hotels top out at around two hundred rooms, but they can have as many as three hundred or so at the outward extreme, as long as staff:guest ratio is high and the hotel is designed in such a way that it does not feel too big. It is virtually impossible for hotels larger than this size to uniformly achieve luxury level service for all guests, in my experience.