The ideal size does not go down. You want the bus as large as practical in all cases. The difference in cost between a small and large bus is not significant

The ideal size does not go down. You want the bus as large as practical in all cases. The difference in cost between a small and large bus is not significant (a little more full, and the bus costs 10x as much - but both of these are insignificant over the life of the bus), and the large bus can handle it if you decide to take everyone bowling after your kids birthday party or some such.