How to size wire gauge for BLDC motor
The current in the motor phase leads is usually greater than the current from the DC supply.  You can calculate the RMS phase current if you know the torque constant \$K_t\$ of the motor and the torque you intend to operate at, and you can calculate \$K_t\$ from the voltage constant \$K_v\$ if you know that.  If you don't know \$K_v\$ you can determine it by measuring the back EMF voltage produced at the motor terminals when it's rotated at a known speed while disconnected from the controller.

Many hobby R/C aircraft use quite aggressively thin wire sizes for both DC and motor phases.  They get away with it because the wiring is exposed on the frame rather than bundled, and has air moving over it.  It's worth it because copper is expensive and heavy!