I am wanting to purchase an air exchanger for my home and don't know what size to get.

When you say air exchanger is it to just recirculate air in your house to filter it or to remove stale air and bring fresh air from the outside? Either or will have a guide line from the manufacture you pick for the square footage of the space you are exchanging. If you are exchanging with fresh air from the outside your heat and air equipment will need to be sized for it.