My wife, two kids (11 & 7) and I cruised on the Carnival Horizon in early January. It's my second Carnival cruise, my wife's third. It was our first time on a "big ship", we sailed with just over 4800 passengers.
Embarkation was a breeze. We had an appointment for 1:00pm. We arrived at 12:45 and was on the ship by 1:30.First problem: when you get on the ship, they are using the central elevators to haul luggage and it's difficult to find your way out of the atrium. End result - about 500 people crammed into a small, hot space. Second problem: when we finally found the buffet, the room is too small for all the guests. We ended up climbing two decks with our plates to find a lounge chair to sit in. Embarkation day is always confusing, but this day was worse by far.
The room in the "family harbor" was great. Slept our family of four easily. The family harbor lounge is not open 24 hours, just during certain times. However, it was nice to eat breakfast there instead of the buffet. No complaints about the room.
It slept our family of four suprisingly comfortably. King size bed, with bunk beds where the couch was. We kept it in that configuration the entire cruise, although I understand you could have the steward set it back into a couch during the day if you'd like.