Hi.
It is 16 cm mid/deep and 40 or 42mm Tweeters. (1.68")
This is in inches: 16/2,5 = 6,4" but 6,5 does fit but you anyway need adapters you can not buy. The ebay ones does not fit or anyway only in the rear.
Yes you do as all speakers are the same.
You complain about the possibility of having to pay maybe €50 to have your headunit changed in a PM to me, but yet you are willing to waste money on changing perfectly good speakers (only one of the rears is a 'little faulty' from what you have told me) - it doesn;t make sense!
Cheers,
Mike