It was a simple question, he asked for a size.
I have Polk Audio DB501 5-Inch Coaxial Speakers installed in the front panels below the door near the hinges. You can see them on this picture, it's a tight fit and I measured several times. 5 1/4" might fit too.
And Polk Audio DXi690 6x9 speakers behind the seats, but they are not visible.
With a 720 Watt Amp, I hear them.