just a quicky , what size speaker will fit in the front and rear of my ELXR8 .. 6.5" or 5.25" ?

or must they be 6" ,

6" is a straight forward fitment. more work but better sound from a 6.5"

99% of the time theres sfa difference between 6" and 6.5", some companies measure from the edges of the speaker basket, others from the edges of the cone, etc

JL Audio VR 6.5" splits fit without mods.

As interceptor said, it depends on the drivers...the main issues are mounting depth and whether or not you need to fabricate a baffle / surround to save cutting out a larger hole.

I currently have Alpine SPX-137A's with a 3mm baffle atm and deadened doors - sound very accurate for what they are...lacking in midbass, but I guess that's what you'd expect from a 5.25" mid. Hoping to look at some custom work to house a pair of SPX-177A woofers to finalise a 3-way front stage (bi-amped mid and tweet with active woofers).