size of standard rear speakers?

[wee_blim]

was reading the very informative guide on sterio installation, but the guy said he used 5x7s in the rear. I cant find any 5x7 coaxial speakers, I need to replace 1 as its blowing and was wonderng if someone could tell me the size of the standard rear speakers. Im not bothered modifiying mounts to fit big speakers, but want a decent speaker thats a straight fit loud enough to drown out exhaust noise on long journeys.

Thanks :WaveBye