You can certainly handle big seas in a small RIB, but how many passengers do you want to take? In lumpy seas, you need proper jockey seats for everyone.
I've had my Ribcraft 4.8 with Mariner 60 4 stroke one season now, mostly for family use (2 adults, 3 children), though on a calm day we took 4 adults and 6 children across the bay in comfort - with two adults sitting on the tubes though. I wanted a smaller RIB with plenty of safe seating, and am very pleased with what I have.
Just posted a couple of pictures to the gallery:
In rougher conditions, a few weekends ago was out in a F6 or so, with 2 adults and 4 children, everyone safe and happy, lots of fun had by all, as described in this thread:http://rib.net/forum/showthread.php?s=&threadid=1039