Just kidding, as a general rule if thumb you want your inlets to be at least 110% of the FSA, if you go above around 120% you'll just start creating drag. The exhaust can be anywhere from 100% to 70% of the FSA. The larger the diameter of your exhaust is the more static thrust you'll have, but you'll lose a bit of top end speed. The smaller your exhaust is the lower your static thrust will be and your top speed will be higher. Smaller exhausts also tend to sound WAY better. Try to eliminate any sharp curves or corners throughout the whole inlet and exhaust system.
Just in case note the FSA stands for Fan Swept Area. This is the area of the fan less the area of the fan hub.
I would only add to what Grifflyer has said to point out that the inlet duct is more sensitive to disturbance than the exhaust duct. Anything that causes a pressure drop (changes in area, sharp angle changes or even a rough surface) simply mean the EDF has to do some work just to get the incoming air back to atmospheric pressure with the result that there is less energy left to speed it up which is what you want.