I like the small to medium sized ships. Seven Seas Mariner at 49,000 GRT and 700 pax is my perfect size although I have liked Seven Seas Navigator at around 30,000 GRT and 490 pax and Seven Seas Voyager at 42,000 GRT and 700 pax. I miss Radisson Diamond. I think she was just a little over 20,000 GRT and 320 pax.
Of course, we are all talking about GRT which really doesn't indicated the mass or weight of the ship; just the enclosed portions of the ship. It is an inexact guide to how much space there is per passenger.