Sizing depends on attic square footage, roof design, and how much net free vent area you already have for intake and exhaust. A fan that is undersized may not move enough air, and a fan installed without adequate soffit intake cannot perform at its rated airflow because it is starved for replacement air.
Carolina Skylights evaluates your attic size, existing intake and exhaust ventilation, and roof exposure first, then recommends the right Solatube solar attic fan model and placement for reliable heat and moisture removal.