If it's like my house, a previous homeowner put in grills the wrong
size and sometimes type. Don't assume that what's in there is
correct. Also, places like Home Depot, Lowes, and Ace only have even
numberes sizes. I.E. - 6 x 10, 8 x 10. if you happen to need
something where one of the sides is an odd number, than you might have
to to go a HVAC supply place and order it. That's what someone did in
my house, went to HD, bought the closest thing, then put it on, then
painted over it (so not only was it the wrong size, it was horribly
ugly from the paint).