I have booked a family of five on the Carnival Triumph in an outside cabin. They brought in a roll away for the fifth person. The cabin steward would put the roll away during the day and set it up in the evening. The Coast Guard regulates how many 3/4/5 passengers can be booked on each sailing. So depending on how many rooms are already booked with 3/4/5 at the time you book you may or may not be able to get it. You may want to check the difference of booking two rooms which would allow you more space.
If you are set on one cabin rather than two adjoining ones, you can fit five in Celebrity Mercury's family ocean view cabins, located, for example at the back of the Panorama Deck. There is the usual king type bed, a sofa bed that opens and a pull down upper berth. You cannot book one cabin over the internet with five, but you can with your TA or with Celebrity direct. I am not sure if any other ships in the Celebrity fleet have this configuration in their family ocean view cabins. I think they are about 210 sq. feet. However, I do not think the difference in cost between that and two adjoining cabins is very large, and you would have much more space and two bathrooms with the adjoining cabin configuration. Hope this helps.
As a PS to what I just posted, I meant connecting (not merely adjoining) cabins, which have a door from one to the other which can be kept opened, much as connecting hotel rooms. I know that the older Royal Caribbean ships, such as Monarch of the Sea (very nicely refurnished) have many connecting cabins.
I booked 5 people in the cheapest interior room on Carnival Victory. You just have to ask which ships will allow a roll away bed. (We got bumped to ocean view) It wasn't cramped at all for us! (Sensation will also allow 5 people)
Sovereign of the Seas, Royal Caribbean 11-17-03
Fascination, CCL 2-14-05
Fantasy, CCL 2-05-06
Elation, CCL 9-24-06
Carnival Victory, CCL 1-28-07
Sovereign of the Seas, Royal Caribbean, 9-24-07