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If size matters!! Generally the newer ships (post 1996) have larger cabins than there older brethren. If quality is what you mean, P&O, RCI, Celebrity, NCL, & Carnival would suit most peoples taste & pocket. The older ships of Thomson, Fred Olsen, Island & the older Costa ships are all pre 1996 were built for a different market.
I have had inside cabins on Thomson Fred Olsen RCI and Celebrity
The Thomson cabin was big enough but needed up grading and the beds were in an "L"
The Olsen cabin on Braemar was small and very noisy (although clean and well looked after) although others on a higher floor seemed happier
The rooms on RCI (brilliance of the seas) and Celebrity (Century) were both on cheaper decks but were first class in every respect