Apart from the exhaust fans, that are necessary for the proper exchange of the air in grow area, circulation fans are also very useful.
They make sure that the air within the box or room is distributed evenly, which is not the case if it is left on its own.
Warm air tends to gather at the top, leading to various temperatures and humidity in different parts of the grow area - something your cannabis plants won't be too pleased about.
In this case you won't need great power - just enough to keep the air moving inside.
A useful thing is a regulation of the fan speed, as well as the automatically rotating head - you don't want the air to be blown all the time in the same direction.
Regulated height might also come handy, as you will want to move it higher as the plants get bigger.
Once inside the grow area, you need to have an impression that the air inside is not still - but make sure that it doesn't cause your cannabis plants to fall over or bend too much!
Different from the exhuast fan that operates at full speed only when the lights are on, circulation fan is supposed to work 24/7 - after all, the wind blows all the time in cannabis natural environment, and indoor cultivation is all about emulating its natural surroundings.