4 inch front, 5 inch rear.
Fronts are fixed in by bent-over metal tabs but these can easily be changed for normal screws/bolts provided you drill out the metal tab slots.
Rear speakers are clamped between two plastic enclosures, this time with four bolts and captive nuts in the plastic.

In both cases the PCD of the bolt/screw holes are a standard size so the normal 'replacement' speakers will fit fine.
HTH.