That's my understanding, too, xmech2k. The intake diameter probably isn't as critical, as long as it is at least the diameter of the fan housing. A bit larger may be desireable. The length of the intake is probably pretty important so that the airflow going into the fan is uniformly distributed over the fan area.
The exhaust outlet probably can be slightly smaller than the fan diameter. This will accelarate the flow, but puts more load on the motor. I have noticed that the nozzles of fighter jets adjust slightly as throttle is applied.