If you will be installing them high in your auditorium, go with a good commercial/industrial fan, they are offered in various sizes 36,48,56,and 60 inch models if my memory is correct. The effective floor coverage is dependent on ceileing height, I believe a 56" fan mounted at 20' covers 1500 sq. feet. If you give me your specs I can check into it for you, or at least point you in the right direction.
Make sure they come with a safety cable as most state electrical codes require them for commercial installations, get metal blades as they will not warp and have a lesser chance of becoming unbalanced, do not go cheap, be sure to get a good motor or it will become a noise maker and annoyance. In Michigan a fan like this has to be hung at least 10' from the floor to meet code.
One other thing is to be sure you get the proper fan speed controller to control the RPM adjustment.
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