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To follow up James Stewart's answer, there must be sufficient mixing to ensure the bacterial floc is maintained in suspension. Having come across an aeration system, which put oxygen into the water but failed because once the floc reached a certain size, it settled, rendering it useless because the bacteria failed to multiply. Full mixing is important to prevent anaerobic growth at the bottom and corners of the tank.
I would select it in fuanción to the size and the depth of the reactor, once you see the whole area efctivated, I go and select what there is in the market that I guarantee lower maintenance and energy consumption, are the variables that at first glance I see morecritics.
Usually in an aeration setup, you are doing so to meet Dissolved Oxygen requirements. Alternatively, you might be monitoring ORP, or COD, or BOD. You will know the right amount by using approximate rules of oxygen introduction, i.e. how much oxygen saturated water at what temperature is being mixed with what pool volume of "dead" water. Are you using induction of air as a means of gaining oxygen saturation? There are many variables to consider, but perhaps, you can do this incrementally if you first do some approximate calculations, install a pump, and observe effects of the initial set up.
We are using induction of air to oxidize BOD in the aeration tank. We plan to do that by introducing a jet aerator which is practically silent and it eliminates the usage of using a fine diffuser along with the piping system that comes with it. But the issue is the criteria with which we select the aerator. What are the criterias other than the air flowrate and tank size which affects the selection of the right aerator.?