I never noticed before that the Bennets needed more than one carriage to get home from the Netherfield ball! At first, I thought the "s" might be a typo in the RoP text, but my copy of P&P says the same thing. As for size, I don't think a carriage which had room for three adults on each seat would be excessively large - that's about the size of the backseat of a modern car, especially if people are not wearing seatbelts.

As for the second carriage, I'm no expert, but I think it would be easy enough to hire a carriage, kind of like a taxi, for special occasions. Remember that Mrs. Long came to the Meryton Assembly in a hack chaise (ch.5)