Depends on the ship. Oceania has some of the top 'mid-sized' ships out their. The companies like Seabourn / Paul Gauguin / et al dominate the 'small-ship' market.
For 'families' I'd say the larger the ship the better. For couples, the smaller definitely.
One of the good things about Oceania is they have some really nice rooms at the top level plus long journeys rather than port to port and back again. Oceania / Carnival / Royal Caribbean are customers of mine. There are people paying like $10k+ a week on some Oceania ships and they never leave the boat.
I was on the Carnival Ecstasy this last cruise. One of the oldest (may be the oldest in their fleet at a 1991 christening by Kathy Lee Gifford

), it was still in nice condition, but dated. Also although a smaller ship, there were too many children on this sailing to enjoy it if you were planning a 'couples only' event. If you wanted to use either of the hot tubs on the ship, they pretty much had 5-10 children in them even past 2am. Even at the only outside drinking bar (there was an outside 'soda' bar as well), some plopped their toddlers/elementary kids on the few seats at the bar available.
That said, I think if I was on this boat with my family; the kids would have a lot of fun and the girl and I would take a day out at Atlantis or one of the hotels in the Bahamas for the day. For the cost, (this cruise was a free bonus for me though); I have known people to get even 5 days on a Carnival cruise for around $200. The drink prices were not that terrible. I had a 'free drink' card, but they still have you sign a receipt. My beers were $5.25 a piece. Many ordering cocktails were also paying about $5-6 a drink. They also have very large drinks at a great value. They do load the ice in drinks that come with it though.