Rear speaker replacement

Ok so i bought some new rear spealers, the guy at the best buy looked it up in his comp and tole me they were 5 1/4 so i bought thoes, i went to install em and they were to small, i measures the old speaker it said 5 1/2. yet i dont belive they sell 5 1/2. is this chevys way of screwing us over and making me make custom mounting brackets?

What year is yours?

I haven't changed mine yet but I was always told that 99+'s were 6 3/4 and you use 6 1/2.

I guess I'll find out what size they are when I tear mine apart