Hook the motor and blower up to 12 V and try to simulate the duct resistance.
Read the amperage.
Let us assume amps was 10 and you want to run at one half speed ie 6V - not exact but close.
So R = E/I = 12/10 = 1.2 ohm
amps should now be less than 1/2 due to affinity law. SO let us assume 5.
Watts = 5 x 6 = 30
This is not exact but should be close and a bit oversized.
This is all true if you had a pure resistive load but you have a motor driving a load whose power demand is the square of the change - you dropped speed by 1/2 so power demand by the fan is 1/8. Double check amperage at 6 V and hopefully I am too conservative.
Dan Bentler
