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.
I had to special order all of mine. Some are odd sizes (maening 5 X
somehting as opposed to 4 X something or 6 X something) and Home Depot
and Lowes don't sell all sizes available. I did have to refurbish two
of mine because I couldn't get them anywhere. I had to soak them in
paint thinner to get off several layers of paint, then primed and
painted with a paint gun. A real nusiance, but worth the effort.