Internally it looks like there is a 2 ohm FET going either to GND or the top battery. The mid point is thus connected either to GND or 8.4V. I would say the worst case power is 4.2^2/33 about 1/2W
So I would go for a 1210. Unless you are super space constrained and need to do better. Then we have to know much more about the balancing operation like what duration or duty cycle of the FET switches is and how that maybe lowers the dissipation in the resistors.
Alternatively maybe you can up the value of the resistors and reduce somewhat the balancing power.