We have seen up to 20 at a long table together. Have sat at a table with 12 a few times. This worked great for us. The adults sat at one end and the kids sat on another. We were able to talk, and they could without interrupting our conversation. It was still easy to just look down the table to check on them and see if they needed anything.
As far as it affecting service to the table if it is larger, I don't see that as a concern and we haven't experienced it either. The servers have around 18 guests each seating, sometimes a couple more. They usually bring out everyone's food at once with the covers on them and set it at the serving station. It doesn't really matter if they have 18 people at three tables or six. Your orders are taken at the same time, the orders are entered into the system at the same time, and you will get your food at the same time. Also, if your party is split up between two tables the servers still treat it as one party. They wouldn't serve everyone at one table and make the other wait.