We are using a transfer service to get from fco to hotel and then to cruise ship and back to fco. How do I know what they consider large, medium, and small suitcases? They said use airline standards, but I don't know what that means. Is a carryon considered small or medium?
I've never known an airline to classify bags as "small," "medium," or "large" - only as "small enough to carry on" and "too big to carry on - must be checked." So, like you, I'm puzzled. You have to ask the transfer service exactly what dimensions they mean for each size.
I would consider carryon as small. And the big, rolling 29", 50 lb, just under the max size allowed by the airlines as large. Medium would be something in middle. As a side note, when we go on cruises we get by very well with three carry on size bags total for two of us. With some careful planning you don't need a huge pile of luggage for a cruise.