Remove the grille fron the wall or ceiling and measure the opening.
Round to the nearest inch. That is the size grille yuo want. The
grille is sized by the openingn not the outside of the flange. It
sounds like you want a 14" x 8". The width is stated first, times the
height. Note that if you get a grille with 1/2 inch between the vanes,
it will be quieter than if it has 1/3 inch between the vanes.