I think it depends on what you are looking for.
Our first cruise was on Carnival Tropical, little did we know, this was the smallest and oldest ship in their fleet, (they had retired the Carnivale) everything else was Fantasy class ships. We had a great time and it started us on a path that now includes over 30 cruises, some still on Carnival though we have moved on to lines we prefer more.
Currently the smallest in Carnivals fleet are the Fantasy class ships, they are still a nice easy to get around in ship. We were on the Spirit a while back and it was a pain to get from here to there, to me the ship just didn't flow....
If you are comfortable with Fantasy class, doesn't matter, they are all the same, just different colors inside. That said, if you want to move up in size, give the grand class Princess ships a look, bigger but the only downside is that it is a long walk from front to back, and yup, there are no elevators that go that direction.
