1- Please take the grill out of the vent and place it on a flat surface

2- Flip the grill and place it upside down so you can see the grill's flange that goes into the opening

3- Using a measuring tape or a ruler, measure the width of the flange. If the size is between 3.5" to  4" ( 89 to 101mm) same as shown in the picture, a 4"X10" device is needed, and if it is between 2.5" to 3" (63 to 76 mm) a 3"X10" devices will fit into the opening.