Take a look at
http://files.wizkid057.com/teslapack/update2/Tesla%20BMS%20M...For cell #1, my guess is that the 4 parallel resistors (R3) are the balancing resistors and that Q1 is a small logic level MOSFET which enables cell balancing.
On a BMS project I'm working on balancing current is only 250mA, so a small SOT-23 MOSFET is all you would need.