Step 1) Measure outside dimensions of your vent
Step 2) Match to appropriate size on chart

Outside Dimensions of Existing VentPurchase Airflow Breeze
W 5.5" x L 11.5"4" x 10" Airflow Breeze
W 5.5" x L 13.5"4" x 12" Airflow Breeze
W 7.75" x L 11.75"6" x 10"/12" Airflow Breeze*
W 7.75" x L 13.75"6" x 10"/12" Airflow Breeze*

* Unit fits both size vent openings