How do I begin to size the MOSFETS for a 3-phase brushless dc motor drive circuit? I know the motor voltage (300V), pwm frequency (20kHz), and motor kW (2.5kW@3500 rpm).
For a 300V motor you probably want a transistor rated at least twice that, or 600V. The motor current for each phase is about 2500/(300 *√3) = 4.8A. You probably need one with at least four times that rating or 20A, or even larger if the motor starting current is high.
In most applications yes.
However if the cost is not too much higher an even higher voltage and current rating is still better.
Plus dont forget to use properly sized and rated snubbers and surge suppression on all power handling devices and circuits.
The odd system noises that a VFD can create while doing its job can do more damage to the switching devices than anything else.