
This is an observation and an opinion only. I believe they go by weight just as much by size. We are a family of 5, 4 adults and 1 child and have always gone on a ride all together. When you think of horse rides anywhere they always ask your weight, even over at FW they make you stand on a scale, even if they can tell you are u dear the 250 maximum. So the horses have a driver, and one I can remember over the years is quiet heavy, another one is quiet regular size. After about our third year, I realized why, but some years they would ask us to wait for the other horse, even though we had an appt, it took a few times to realize, they were making us wait because of our total weight, no they don't make you stand on a scale but they can look at a party and gather oh they weigh about 650 together and I the driver weight 150 so we are good at 800 pounds. Which is find for most horse drawn horses, 800 to 1000 pounds because they also have the wagon weight.
So with that as a guide, if you walk up and each of the female adults weigh 125, and each of the men weigh 180, and each of the kids weigh 40, and you all fit most likely they will allow you all in a carriage.
On the other hand if one person weighs 300, 2 people weigh 200 and one person weighs 180 and the kids both weigh 65, and you get a heavy set driver, you most likely will be ask to seperate the group.
Since that is so complicated to write, Disney tries to come up with the average, and say 4 adults or 2 adults, three small children.
Picture below shows us all before our weight lose program, I would say we are a good 130 pounds less between us now, oh except the kids they have grown over the past few years. This was a carriage ride out at FW. And sorry, no picture of service dog, they will not let him ride.