I just got back from a GAP tour in Costa Rica last month. Our group had 13 people on it and we all seemed to get along pretty well. The good thing I found about GAP was that while the accommodations, transfers and some meals were included in the price, the optional excursions were not. This way you are able to pick and choose what you want to do. Its a group tour but at the same time if you don't want to do something you don't have to.
One guy on our trip did two tours back to back, his first group was only 4 people which he said was not has fun, so it might be better to go with one that is more booked up. almost 10 years ago