I would think 50 - 60 foot minimum and probably higher. I think I read where Oasis hit 65 ft seas on its way to the US.
Highest we have ever encountered was 15 ft leaving Panama. It did make some sick, but overall was not that bad.
Carnival Celebration 2003
Carnival Elation 2005
Carnival Conquest 2007
Royal Caribbean Rhapsody of the Seas 2009
Carnival Freedom 2010