900 square feet? wow thats big. You will need a very large filter to cover that much area. You should pick your filter based on the size of the room then pick the fan based on the cfm rating of the filter. Based on that much square footage I believe you are going to need one of those monstrously huge stand up filters, like the phresh 14" by 50" which is only rated for 3,800 cfm. Your area 900 square feet x height (7?) would be like 6,300 cubic feet. Based on that filter you will need a fan that can move a hell of a lot of air. If you are getting a filter and fan that big it doesn't matter where you put it, it will suck everything up. Usually you would put it up high to pull out the hot air, but if you are not exhausting this air it doesn't matter where the filter is.