I have one and she is wonderful! In general, they are slightly larger than green cheeks, but not by a whole lot. According to Avianweb.com, the weight range for CBCs is 79-94 grams while the range for GCCs is 60-80 grams. Penny weighed in at 86 grams at her last vet visit.
In general, I would say that Penny isn't a super cuddly bird, although she does like to have her head scratched. She is very sweet and usually not very nippy at all. The only time she generally gets nippy is when she meets new people, but she usually warms up to them pretty quickly. She is a little bit more laid back than the green cheeks I've encountered, but she's still a conure and has plenty of energy.
I would say that Penny is a very quiet bird in general. I used to have cockatiels and a lovebird, and she is way quieter than they were. I've heard people say that they are louder than green cheeks, but I haven't personally experienced that.