1. Start by measuring the hole in the vent — do not measure the actual grille you're replacing as they overlap in the front and are always larger than the vent.
For example, if your hole is 14 x 20, you need a 14 x 20 grille. The front-end overlapping section of the grille is approximately 2 inches wider.
2. In summary, you should first measure the width of the grille, followed by the height.
To see the grilles we have on offer for your wall, check out this page.